Learn About Hard Disk Drives

In this episode of the I Can't Sleep Podcast, fall asleep learning about Hard Disk Drives. I'm not going to try and come up with something interesting for this one because it was a bore to read. Happy sleeping!

Transcript

Welcome to the I Can't Sleep Podcast,

Where I read random articles from across the web to bore you to sleep with my soothing voice.

I'm your host,

Benjamin Boster.

Today's episode is from a Wikipedia article titled,

Hard Disk Drive.

A hard disk drive,

HDD,

Hard disk,

Hard drive,

Or fixed disk,

Is an electromechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magnetic material.

The platters are paired with magnetic heads,

Usually arranged on a moving actuator arm,

Which read and write data to the platter surfaces.

Data is accessed in a random access manner,

Meaning that individual blocks of data can be stored and retrieved in any order.

HDDs are a type of non-volatile storage,

Retaining stored data when powered off.

Modern HDDs are typically in the form of a small rectangular box.

Hard disk drives were introduced by IBM in 1956,

And were the dominant secondary storage device for general purpose computers beginning in the early 1960s.

Hard disk drives maintain this position into the modern era of servers and personal computers,

Though personal computing devices produced in large volume like mobile phones and tablets rely on flash memory storage devices.

More than 224 companies have produced HDDs historically,

Though after extensive industry consolidation,

Most units are manufactured by Seagate,

Toshiba,

And Western Digital.

HDDs dominate the volume of storage produced exabytes per year for servers.

Though production is growing slowly by exabytes shipped,

Sales revenues and unit shipments are declining because solid state drives,

SSDs,

Have higher data transfer rates,

Higher aerial storage density,

Somewhat better reliability,

And much lower latency and access times.

The revenues for SSDs,

Most of which use NAND flash memory,

Slightly exceeded those for HDDs in 2018.

Flash storage products had more than twice the revenue of hard disk drives as of 2017.

Though SSDs have four to nine times higher cost per bit,

They are replacing HDDs in applications where speed,

Power consumption,

Small size,

High capacity,

And durability are important.

As of 2019,

The cost per bit of SSDs is falling and price premium over HDDs has narrowed.

The primary characteristics of an HDD are its capacity and performance.

Capacity is specified in unit prefixes corresponding to powers of 1,

000.

A 1TB TB drive has a capacity of 1,

000GB,

Where 1GB equals 1,

000MB,

Which equals 1,

000,

000KB,

Which equals 1,

000,

000,

000,

000,

000,

000,

000,

000,

000,

000,

000,

000,

000,

000,

000,

000,

000,

000,

000,

000.

Typically,

Some of an HDD's capacity is unavailable to the user because it is used by the file system and the computer operating system,

And possibly in-build redundancy for error correction and recovery.

There can be confusion regarding storage capacity,

Since capacities are stated in decimal gigabytes by HDD manufacturers,

Whereas the most commonly used operating systems report capacities and powers of 1,

024,

Which results in a smaller number than advertised.

Performance is specified as time required to move the heads to a track or cylinder,

Average access time,

The time it takes for the desired sector to move under the head,

Average latency,

Which is a function of the physical rotational speed in revolutions per minute,

And finally the speed at which the data is transmitted,

Data rate.

The two most common form factors for modern HDDs are 3.

5-inch for desktop computers and 2.

5-inch primarily for laptops.

HDDs are connected to systems by standard interface cables,

Such as SATA,

Serial ATA,

USB,

SAS,

Serial Attached SCSI,

Or PETA,

Parallel ATA cables.

The first production IBM hard disk drive,

The 350 disk storage,

Shipped in 1957 as a component of the IBM 305 RAMAC system.

It was approximately the size of two large refrigerators and stored 5,

000,

000 6-bit characters 3.

7 megabytes on a stack of 52 disks,

100 surfaces used.

The 350 had a single arm with two read-write heads,

One facing up and the other down,

That moved both horizontally between a pair of adjacent platters and vertically from one pair of platters to a second set.

Variants of the IBM 350 were the IBM 355,

IBM 7300,

And IBM 1405.

In 1961,

IBM announced and in 1962 shipped the IBM 1301 disk storage unit,

Which superseded the IBM 350 and similar drives.

The 1301 consisted of one for Model 1 or two for Model 2 modules,

Each containing 25 platters,

Each platter about an eighth of an inch thick,

And 24 inches in diameter.

While the earlier IBM disk drives used only two read-write heads per arm,

The 1301 used an array of 48 heads,

Comb each array moving horizontally as a single unit,

One head per surface used.

Cylinder mode read-write operations were supported,

And the heads flew about 250 micro-inches above the platter surface.

Motion of the head array depended upon a binary adder system of hydraulic actuators,

Which assured repeatable positioning.

The 1305 cabinet was about the size of three large refrigerators placed side-by-side,

Storing the equivalent of about 21 million 8-bit bytes per module.

Access time was about a quarter of a second.

Also in 1962,

IBM introduced the Model 1311 disk drive,

Which was about the size of a washing machine and stored two million characters on a removable disk pack.

Users could buy additional packs and interchange them as needed,

Much like reels of magnetic tape.

Computer models of removable pack drives,

From IBM and others,

Became the norm in most computer installations and reached capacities of 300 megabytes by the early 1980s.

Non-removable HTDs were called fixed-disk drives.

In 1963,

IBM introduced the 1302,

With twice the track capacity and twice as many tracks per cylinder as the 1301.

The 1302 had one for Model 1 or two for Model 2 modules,

Each containing a separate comb for the first 250 tracks and the last 250 tracks.

Some high-performance HTDs were manufactured with one head per track,

E.

G.

Burroughs B475 in 1964,

IBM 2305 in 1970,

So that no time was lost physically moving the heads to a track and the only latency was the time for the desired block of data to rotate into position under the head.

Known as fixed-head or head-per-track disk drives,

They were very expensive and are no longer in production.

In 1973,

IBM introduced a new type of HTD,

Codenamed Winchester.

Its primary distinguishing feature was that the disk heads were not withdrawn completely from the stack of disk platters when the drive was powered down.

Instead,

The heads were allowed to land on a special area of the disk surface upon spin-down,

Taking off again when the disk was later powered on.

This greatly reduced the cost of the head actuator mechanism,

But precluded removing just the disks from the drive,

As was done with the disk packs of the day.

Instead,

The first models of Winchester technology drives featured a removable disk module,

Which included both the disk pack and the head assembly,

Leaving the actuator motor in the drive upon removal.

Later,

Winchester drives abandoned the removable media concept and returned to non-removable platters.

In 1974,

IBM introduced the swinging arm actuator,

Made feasible because the Winchester recording heads functioned well when skewed to the recorded tracks.

The simple design of the IBM GV Gulliver drive,

Invented at IBM's UK Hursley lab,

Became IBM's most licensed electromechanical invention of all time,

The actuator and filtration system being adopted in the 1980s,

Eventually for all HTDs,

And still universal nearly 40 years and 10 billion arms later.

Like the first removable pack drive,

The first Winchester drives used platters 14 inches in diameter.

In 1978,

IBM introduced a swing arm drive,

The IBM 0680 Piccolo,

With 8 inch platters,

Exploring the possibility that smaller platters might offer advantages.

Other 8 inch drives followed,

Then 5 1⁄4 inch drives,

Sized to replace the contemporary floppy disk drives.

The latter were primarily intended for the then-fledgling personal computer PC market.

Over time,

As recorded densities were greatly increased,

Further reductions in disk diameter to 3.

5 inches and 2.

5 inches were found to be optimum.

Powerful rare-earth magnet materials became affordable during this period,

And were complementary to the swing arm actuator design to make possible the compact form factors of modern HTDs.

As the 1980s began,

HTDs were a rare and very expensive additional feature in PCs,

But by the late 1980s their cost had been reduced to the point where they were standard on all but the cheapest computers.

Most HTDs in the early 1980s were sold to PC end-users as an external add-on subsystem.

The subsystem was not sold under the drive manufacturer's name,

But under the subsystem manufacturer's name,

Such as Corva Systems and Tallgrass Technologies,

Or under the PC system manufacturer's name,

Such as the Apple Profile.

The IBM PC XT in 1983 included an internal 10 megabit HTD,

And soon thereafter internal HTDs proliferated on personal computers.

External hard disk drives remained popular for much longer on the Apple Macintosh.

Many Macintosh computers made between 1986 and 1998 featured a CSCI port on the back,

Making external expansion simple.

Older compact Macintosh computers did not have user-accessible hard drive bays.

Indeed,

The Macintosh 128K,

Macintosh 512K,

And Macintosh Plus did not feature a hard drive bay at all.

So on those models,

External CSCI disks were the only reasonable option for expanding upon any internal storage.

HTD improvements have been driven by increasing aerial density.

Applications expanded through the 2000s,

From the mainframe computers of the late 1950s to most mass storage applications,

Including computers and consumer applications such as storage of entertainment content.

In the 2000s and 2010s,

NAND began splanting HTDs in applications requiring portability or high performance.

NAND performance is improving faster than HTDs,

And applications for HTDs are eroding.

In 2018,

The largest hard drive had a capacity of 15 terabytes,

While the largest capacity solid-state drive had a capacity of 100 terabytes.

As of 2018,

HTDs were forecast to reach 100 terabytes capacity around 2025.

But as of 2019,

The expected pace of improvement was pared back to 50 terabytes by 2026.

Smaller form factors,

1.

8 inches and below,

Were discontinued around 2010.

The cost of solid-state storage,

NAND,

Represented by Moore's Law,

Is improving faster than HTDs.

NAND has a higher price elasticity of demand than HTDs,

And this drives market growth.

During the late 2000s and 2010s,

The product lifecycle of HTDs entered a mature phase,

And slowing sales may indicate the onset of the declining phase.

The 2011 Thailand floods damaged the manufacturing plants and impacted hard disk drive costs adversely between 2011 and 2013.

In 2019,

Western Digital closed its last Malaysian HTD factory due to decreasing demand to focus on SSD production.

All three remaining HTD manufacturers have had decreasing demand for their HTDs since 2014.

A modern HTD records data by magnetizing a thin film of ferromagnetic material on both sides of a disk.

Sequential changes in the direction of magnetization represent binary data bits.

The data is read from the disk by detecting the transitions in magnetization.

Other data is encoded using an encoding scheme,

Such as Run Lengths Limited Encoding,

Which determines how the data is represented by the magnetic transitions.

A typical HTD design consists of a spindle that holds flat circular disks,

Called platters,

Which hold the recorded data.

The platters are made from a non-magnetic material,

Usually aluminum alloy,

Glass,

Or ceramic.

They are coated with a shallow layer of magnetic material,

Typically 10-20 nm in depth,

With an outer layer of carbon for protection.

For reference,

A standard piece of copy paper is 0.

07-0.

18 nm thick.

The platters in contemporary HTDs are spun at speeds varying from 4,

200 RPM in energy-efficient portable devices to 15,

000 RPM for high-performance servers.

The first HTDs spun at 1,

200 RPM,

And for many years 3,

600 RPM was the norm.

As of November 2019,

The platters in most consumer-grade HTDs spin at 5,

400 or 7,

200 RPM.

Information is written to and read from a platter as it rotates past devices called read and write heads that are positioned to operate very close to the magnetic surface,

With their flying height often in the range of tens of nanometers.

The read and write head is used to detect and modify the magnetization of the material passing immediately under it.

In modern drives,

There is one head for each magnet platter surface on the spindle,

Mounted on a common arm.

An actuator arm,

Or access arm,

Moves the heads on an arc,

Roughly radially across the platters as they spin,

Allowing each head to access almost the entire surface of the platter as it spins.

The arm is moved using a voice coil actuator,

Or in some older designs,

A stepper motor.

Early hard disk drives wrote data at some constant bits per second,

Resulting in all tracks having the same amount of data per track,

But modern drives since the 1990s use zone bit recording,

Increasing the write speed from inner to outer zone,

And thereby storing more data per track in the outer zones.

In modern drives,

The small size of the magnetic regions creates the danger that their magnetic state might be lost because of thermal effects,

Thermally induced magnetic instability,

Which is commonly known as the superparamagnetic limit.

To counter this,

The platters are coated with two parallel magnetic layers,

Separated by a three-atom layer of the non-magnetic element ruthenium,

And the two layers are magnetized in opposite orientation,

Thus reinforcing each other.

Another technology used to overcome thermal effects to allow greater recording densities is perpendicular recording,

PMR,

First shipped in 2005 and as of 2007 used in certain HTDs.

Perpendicular recording may be accomplished by changes in the manufacturing of the read-write heads to increase the strength of the magnetic field created by the heads.

In 2004,

A higher-density recording media was introduced consisting of coupled soft and hard magnetic layers.

So-called exchange-spring media magnetic storage technology,

Also known as exchange-coupled composite media,

Allows good writeability due to the write-assist nature of the soft layer.

However,

The thermal stability is determined only by the hardest layer and not influenced by the soft layer.

Fixed-control MAMR,

FCMAMR,

Allows a hard drive to have increased recording capacity without the need for new hard-disk drive platter materials.

The MAMR hard drives have a microwave-generating spin-torque generator STO on the read-write heads,

Which allows physically smaller bits to be recorded to the platters,

Increasing aerial density.

Typically,

Hard drive recording heads have a pole,

Called a main pole,

That is used for writing to the platters.

And adjacent to this pole is an air gap and a shield.

The right coil of the head surrounds the pole.

The STO device is placed in the air gap between the pole and the shield to increase the strength of the magnetic field created by the pole.

FCMAMR technically doesn't use microwaves,

But uses technology employed in MAMR.

The STO has a field generation layer,

FGL,

And a spin injection layer,

SIL,

And the FGL produces a magnetic field using spin-polarized electrons originating in the SIL,

Which is a form of spin-torque energy.

A typical HTD has two electric motors,

A spindle motor that spins the disks and an actuator motor that positions the read-write head assembly across the spinning disks.

The disk motor has an external rotor attached to the disks.

The stator windings are fixed in place.

Opposite the actuator,

At the end of the head support arm,

Is the read-write head.

Thin printed circuit cables connect the read-write heads to amplifier electronics mounted at the pivot of the actuator.

The head support arm is very light,

But also stiff.

In modern drives,

Acceleration at the head reaches 550 grams.

The actuator is a permanent magnet and moving coil motor that swings the heads to the desired position.

A metal plate supports a squat neodymium iron boron NIB high-flux magnet.

Beneath this plate is the moving coil,

Often referred to as the voice coil by analogy to the coil in loudspeakers,

Which is attached to the actuator hub,

And beneath that is a second NIB magnet,

Mounted on the bottom plate of the motor.

Some drives have only one magnet.

The voice coil itself is shaped rather like an arrowhead and is made of doubly coated copper magnet wire.

The inner layer is insulation and the outer is thermoplastic,

Which bonds the coil together after it is wound on a form,

Making it self-supporting.

Portions of the coil along the two sides of the arrowhead,

Which point to the center of the actuator bearing,

Then interact with the magnetic field of the fixed magnet.

Current flowing radially outward along one side of the arrowhead and radially inward on the other produces the tangential force.

If the magnetic field were uniform,

Each side would generate opposing forces that would cancel each other out.

Therefore the surface of the magnet is half North Pole and half South Pole,

With the radial dividing line in the middle,

Causing the two sides of the coil to see opposite magnetic fields and produce forces that add instead of canceling.

Currents along the top and bottom of the coil produce radial forces that do not rotate the head.

The HTD's electronics control the movement of the actuator and the rotation of the disc and transfers data to and from a disc controller through a buffered interface.

Feedback of the drive electronics is accomplished by means of special segments of the disc dedicated to servo feedback.

These are either complete concentric circles in the case of dedicated servo technology or segments interspersed with real data in the case of embedded servo,

Otherwise known as sector servo technology.

The servo feedback optimizes the signal-to-noise ratio of the GMR sensors by adjusting the voice coil motor to rotate the arm.

A more modern servo system also employs Milla and or micro actuators to more accurately position the read-write heads.

The spinning of the discs uses fluid bearing spindle motors.

Modern disc firmware is capable of scheduling reads and writes efficiently on the platter surfaces and remapping sectors of the media that have failed.

Modern drives make extensive use of error correction codes,

ECCs,

Particularly Reed-Solomon error correction.

These techniques store extra bits determined by mathematical formulas for each block of data.

The extra bits allow many errors to be corrected invisibly.

The extra bits themselves take up space on the HTD but allow higher recording densities to be employed without causing uncorrectable errors,

Resulting in much larger storage capacity.

For example,

A typical 1TB hard drive with 512-byte sectors provides additional capacity of about 93GB for the ECC data.

In the newest drives as of 2009,

Low-density parity check codes LDPC were supplanting Reed-Solomon.

LDPC codes enable performance close to the Shannon limit and thus provide the highest storage density available.

Typical hard disk drives attempt to remap the data in a physical sector that is failing to a spare physical sector provided by the drive's spare sector pool,

Also called reserve pool,

While relying on the ECC to recover stored data while the number of errors in a bad sector is still low enough.

The SMART,

Self-Monitoring Analysis and Reporting Technology feature,

Counts the total number of errors in the entire HTD fixed by ECC,

Although not on all hard drives as the related SMART attributes,

Hardware ECC Recovered and Soft ECC Correction are not consistently supported and the total number of performed sector remappings as the occurrence of many such errors may predict an HTD failure.

The NoID format developed by IBM in the mid-1990s contains information about which sectors are bad and where remapped sectors have been located.

Only a tiny fraction of the detected errors end up as not correctable.

Examples of specified uncorrected bit read error rates include,

2013 specifications for enterprise SAS disk drives state the error rate to be 1 uncorrected bit read error in every 10 to the 16th bits read.

2018 specifications for consumer SATA hard drives state the error rate to be 1 uncorrected bit read error in every 10 to the 14th bits.

Within a given manufacturer's model,

The uncorrected bit error rate is typically the same regardless of capacity of the drive.

The worst type of errors are silent data corruptions,

Which are errors undetected by the disk firmware or the host operating system.

Some of these errors may be caused by hard disk drive malfunctions,

While others originate elsewhere in the connection between the drive and the host.

The rate of aerial density advancement was similar to Moore's law,

Doubling every two years through 2010,

60% per year during 1988-1996,

100% during 1996-2003,

And 30% during 2003-2010.

Speaking in 1997,

Gordon Moore called the increase flabbergasting,

While observing later that the growth cannot continue forever.

Price improvement decelerated to negative 12% per year during 2010-2017 as the growth of aerial density slowed.

The rate of advancement for aerial density slowed to 10% per year during 2010-2016,

And there was difficulty in migrating from perpendicular recording to newer technologies.

As bit cell size decreases,

More data can be put onto a single drive platter.

In 2013,

A production desktop 3TB HDD with 4 platters would have had an aerial density of about 500 gigabit per inch squared,

Which would have amounted to a bit cell comprising about 18 magnetic grains.

Since the mid-2000s,

Aerial density progress has been challenged by a superparamagnetic trilemma involving grain size,

Grain magnetic strength,

And ability of the head to ride.

In order to maintain acceptable signal-to-noise,

Smaller grains are required.

Smaller grains may self-reverse electrothermal instability unless their magnetic strength is increased,

But known right-head materials are unable to generate a strong enough magnetic field sufficient to right the medium in the increasingly smaller space taken by grains.

Magnetic storage technologies are being developed to address this trilemma,

And compete with flash memory-based solid-state drives,

SSDs.

In 2013,

Seagate introduced Shingled Magnetic Recording,

SMR,

Intended as something of a stopgap technology between PMR and Seagate's intended successor Heat-Assisted Magnetic Recording,

HAMR.

SMR utilizes overlapping tracks for increased data density,

At the cost of design complexity and lower data access speeds,

Particularly write speeds and random access 4K speeds.

By contrast,

HTST,

Now part of Western Digital,

Focused on developing ways to see helium-filled drives instead of the usual filtered air.

Since turbulence and friction are reduced,

Higher aerial densities can be achieved due to using a smaller track width,

And the energy dissipated due to friction is lower as well,

Resulting in a lower power draw.

Furthermore,

More platters can be fit into the same enclosure space,

Although helium gas is notoriously difficult to prevent escaping.

Thus,

Helium drives are completely sealed and do not have a breather port,

Unlike their air-filled counterparts.

Other recording technologies are either under research or have been commercially implemented to increase aerial density,

Including Seagate's Heat-Assisted Magnetic Recording,

HAMR.

HAMR requires different architecture with redesigned media and read-write heads,

New lasers,

And new near-field optical transducers.

HAMR is expected to ship commercially in late 2020 or 2021.

Technical issues delayed the introduction of HAMR by a decade from earlier projections of 2009,

2015,

2016,

And the first half of 2019.

Some drivers have adopted dual independent actuator arms to increase read-write speeds and compete with SSDs.

HAMR's planned successor,

Bit-Pattern Recording,

PPR,

Has been removed from the roadmaps of Western Digital and Seagate.

Western Digital's Microwave-Assisted Magnetic Recording,

MAMR,

Also referred to as Energy-Assisted Magnetic Recording,

EMR,

Was sampled in 2020 with the first EMR drive,

The UltraStar HC-550,

Shipping in late 2020.

Two-dimensional magnetic recording,

TDMR,

And current perpendicular-to-plane giant magnetoresistance CPP-GMR heads have appeared in research papers.

A 3D actuated vacuum drive,

3DHD concept,

And 3D magnetic recording have been proposed.

Depending on assumptions on feasibility and timing of these technologies,

Seagate forecasts that aerial density will grow 20% per year during 2020-2034.

The highest capacity hard disk drives shipping commercially in 2024 are 32TB.

The capacity of a hard disk drive,

As reported by an operating system to the end user,

Is smaller than the amount stated by the manufacturer for several reasons,

E.

G.

The operating system using some space,

Use of some space for data redundancy,

Space use for file system structures.

Confusion of decimal prefixes and binary prefixes can also lead to errors.

Modern hard disk drives appear to their host controller as a contiguous set of logical blocks,

And the gross drive capacity is calculated by multiplying the number of blocks by the block size.

This information is available from the manufacturer's product specification and from the drive itself through use of operating system functions that invoke low-level drive commands.

Older IBM incompatible drives,

E.

G.

IBM 3390,

Using the CKD record format,

Have variable length records.

Such drive capacity calculations must take into account the characteristics of the records.

Some newer DASDs simulate CKD,

And the same capacity formulae apply.

The gross capacity of older sector-oriented HDDs is calculated as the product of the number of cylinders per recording zone,

The number of bytes per sector,

Most commonly 512,

And the count of zones of the drive.

Some modern SATA drives also report cylinder head sector CHS capacities,

But these are not physical parameters because the reported values are constrained by historic operating system interfaces.

The CHS scheme has been replaced by Logical Block Addressing,

LBA,

A simple linear addressing scheme that locates blocks by an integer index,

Which starts at LBA 0 for the first block and increments thereafter.

When using the CHS method to describe modern large drives,

The number of heads is often set to 64,

Although a typical modern hard disk drive is between 1 and 4 platters.

In modern HDDs,

Spare capacity for defect management is not included in the published capacity.

However,

In many early HDDs,

A certain number of sectors were reserved as spares,

Thereby reducing the capacity available to the operating system.

Furthermore,

Many HDDs store their firmware in a reserved service zone,

Which is typically not accessible by the user and is not included in the capacity calculation.

For RAID subsystems,

Data integrity and fault tolerance requirements also reduce the realized capacity.

For example,

A RAID 1 array has about half the total capacity as a result of data mirroring,

While a RAID 5 array with n drives loses 1 over n of capacity,

Which equals to the capacity of a single drive due to storing parity information.

RAID subsystems are multiple drives that appear to be one drive or more drives to the user but provide fault tolerance.

Most RAID vendors use checksums to improve data integrity at the block level.

Some vendors design systems using HDDs with sectors of 520 bytes to contain 512 bytes of user data and 8 checksum bytes,

Or by using separate 512-byte sectors for the checksum data.

Some systems may use hidden partitions for system recovery,

Reducing the capacity available to the end user without knowledge of special disk partitioning utilities like Diskpart and Windows.

Data is stored on a hard drive in a series of logical blocks.

Each block is delimited by markers identifying its start and end,

Error detecting and correcting information,

And space between blocks to allow for minor timing variations.

These blocks often contain 512 bytes of usable data,

But other sizes have been used.

As drive density increased,

An initiative known as Advanced Format extended the block size to 4,

096 bytes of usable data,

With a resulting significant reduction in the amount of disk space used for block headers,

Error checking data,

And spacing.

The process of initializing these logical blocks on the physical disk platters is called low-level formatting,

Which is usually performed at the factory and is not normally changed in the field.

High-level formatting writes data structures used by the operating system to organize data files on the disk.

This includes writing partition and file system structures into selected logical blocks.

For example,

Some of the disk space will be used to hold a directory of disk file names and a list of logical blocks associated with a particular file.

Examples of partition mapping scheme include Master Boot Record,

MBR,

And GUID Partition Table,

GPT.

Examples of data structures stored on disk to retrieve files include the File Allocation Table,

FAT,

In the DOS file system and in ODEs in many UNIX file systems,

As well as other operating system data structures,

Also known as metadata.

As a consequence,

Not all the space on an HTD is available for user files,

But this system overhead is usually small compared with user data.

In the early days of computing,

The total capacity of HTDs was specified in seven to nine decimal digits,

Frequently truncated with the idiom millions.

By the 1970s,

The total capacity of HTDs was given by manufacturers using SI decimal prefixes such as megabytes.

One megabyte equals one million bytes,

Gigabytes.

One gigabyte equals one billion bytes,

And terabytes.

One terabyte equals one trillion bytes.

However,

Capacities of memory are usually quoted using a binary interpretation of the prefixes,

I.

E.

Using powers of 1024 instead of 1000.

Software reports hard disk drive or memory capacity in different forms using either decimal or binary prefixes.

The Microsoft Windows family of operating systems uses the binary convention when reporting storage capacity,

So an HTD offered by its manufacturer as a one terabyte drive is reported by those operating systems as a 931 gigabit drive.

Mac OS X 10.

6 Snow Leopard uses decimal convention when reporting HTD capacity.

The default behavior of the df command line utility on Linux is to report the HTD capacity as a number of 1024 byte units.

The difference between the decimal and binary prefix interpretation caused some consumer confusion and led to class action suits against HTD manufacturers.

The plaintiffs argued that the use of decimal prefixes effectively misled consumers,

While the defendants denied any wrongdoing or liability,

Asserting that their marketing and advertising complied in all respects with the law and that no class member sustained any damages or injuries.

In 2020,

A California court ruled that the use of the decimal prefixes with a decimal meaning was not misleading.

IBM's first hard disk drive,

The IBM 350,

Used a stack of 50 24-inch platters,

Stored 3.

7 megabytes of data,

Approximately the size of one modern digital picture,

And was of a size comparable to two large refrigerators.

In 1962,

IBM introduced its model 1311 disk,

Which used six 14-inch normal-sized platters in a removable pack and was roughly the size of a washing machine.

This became a standard platter size for many years,

Used also by other manufacturers.

The IBM 2314 used platters of the same size in an 11-high pack and introduced the drive in a drawer layout,

Sometimes called the pizza oven,

Although the drawer was not the complete drive.

Into the 1970s,

Hard disk drives were offered to standalone cabinets of varying dimensions,

Containing from one to four hard disk drives.

Beginning in the late 1960s,

Drives were offered that fit entirely into a chassis that would mount in a 19-inch rack.

Models RK05 and RL01 were early examples using single 14-inch platters in removable packs,

The entire drive fitting in a 10.

5-inch high rack space,

Six rack units.

In the mid-to-late 1980s,

The similarly sized Fujitsu Eagle,

Which used,

Coincidentally,

10.

5-inch platters,

Was a popular product.

With increasing sales of microcomputers having built-in floppy disk drives,

FDDs,

HTDs that would fit to the FDD mountings became desirable.

Starting with the ShoeGuard Associates SA1000,

HTD form factors initially followed those of 8-inch,

5.

25-inch,

And 3.

5-inch floppy disk drives.

Although referred to by these nominal sizes,

The actual sizes for these three drives,

Respectively,

Are 9.

5-inch,

5.

75-inch,

And 4-inch wide.

Because there were no smaller floppy disk drives,

Smaller HTD form factors,

Such as the 2.

5-inch drives,

Actually 2.

75-inches wide,

Developed from product offerings or industry standards.

As of 2019,

2.

5-inch and 3.

5-inch hard disks are the most popular sizes.

By 2009,

All manufacturers had discontinued the development of new products for the 1.

3-inch,

1-inch,

And 0.

85-inch form factors due to falling prices of flash memory,

Which has no moving parts.

While nominal sizes are in inches,

Actual dimensions are specified in millimeters.

Reviews

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