ARDI BLOG

A typical hard disk drive has two electric motors; a disk 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 (near center in photo); thin printed-circuit cables connect the read-write heads to amplifier electronics mounted at the pivot of the actuator. A flexible, somewhat U-shaped, ribbon cable, seen edge-on below and to the left of the actuator arm continues the connection to the controller board on the opposite side.
The head support arm is very light, but also stiff; in modern drives, acceleration at the head reaches 550 g.
The silver-colored structure at the lower left of the first image is the top plate of the actuator, a permanent-magnet and moving coil motor that swings the heads to the desired position (it is shown removed in the second image). The 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 only have one magnet).

A disassembled and labeled 1997 hard drive. All major components were placed on a mirror, which created the symmetrical reflections.

The voice coil itself is shaped rather like an arrowhead, and 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. The portions of the coil along the two sides of the arrowhead (which point to the actuator bearing center) interact with the magnetic field, developing a tangential force that rotates the actuator. 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 N pole, half S 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.

[edit] Actuation of moving arm

This section needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (November 2010)

Head stack with an actuator coil on the left and read/write heads on the right

The hard drive's electronics control the movement of the actuator and the rotation of the disk, and perform reads and writes on demand from the disk controller. Feedback of the drive electronics is accomplished by means of special segments of the disk 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 technology). The servo feedback optimizes the signal to noise ratio of the GMR sensors by adjusting the voice-coil of the actuated arm. The spinning of the disk also uses a servo motor. Modern disk firmware is capable of scheduling reads and writes efficiently on the platter surfaces and remapping sectors of the media which have failed.

[edit] Error handling

Modern drives make extensive use of Error Correcting 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 hard drive, but allow higher recording densities to be employed without causing uncorrectable errors, resulting in much larger storage capacity.^[25] In the newest drives 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.^[26]
Typical hard drives attempt to "remap" the data in a physical sector that is failing to a spare physical sector—hopefully while the errors in the bad sector are still few enough that the ECC can recover the data without loss. The S.M.A.R.T. system counts the total number of errors in the entire hard drive fixed by ECC and the total number of remappings, as the occurrence of many such errors may predict hard drive failure.

[edit] Future development

Due to bit-flipping errors and other issues, perpendicular recording densities may be supplanted by other magnetic recording technologies. Toshiba is promoting bit-patterned recording (BPR),^[27] while Xyratex is developing heat-assisted magnetic recording (HAMR).^[28]
October 2011: TDK has developed a special laser that heats up a hard's disk's surface with a precision of a few dozen nanometers. TDK also used the new material in the magnetic head and redesigned its structure to expand the recording density. This new technology apparently makes it possible to store one terabyte on one platter and for the initial hard drive TDK plans to include two platters.^[29]

[edit] Capacity

The capacity of an HDD may appear to the end user to be a different amount than the amount stated by a drive or system manufacturer due to amongst other things, different units of measuring capacity, capacity consumed in formatting the drive for use by an operating system and/or redundancy.

[edit] Units of storage capacity

See also: Binary prefix

Advertised capacity by manufacturer (using decimal multiples)		Expected capacity by consumers in class action (using binary multiples)		Reported capacity
				Windows (using binary multiples)	Mac OS X 10.6+ (using decimal multiples)
With prefix	Bytes	Bytes	Diff.	Windows (using binary multiples)	Mac OS X 10.6+ (using decimal multiples)
100 MB	100,000,000	104,857,600	4.86%	95.4 MB	100.0 MB
100 GB	100,000,000,000	107,374,182,400	7.37%	93.1 GB, 95,367 MB	100.00 GB
1 TB	1,000,000,000,000	1,099,511,627,776	9.95%	931 GB, 953,674 MB	1000.00 GB, 1000,000 MB

The capacity of hard disk drives is given by manufacturers in megabytes (1 MB = 1,000,000 bytes), gigabytes (1 GB = 1,000,000,000 bytes) or terabytes (1 TB = 1,000,000,000,000 bytes).^[30]^[31] This numbering convention, where prefixes like mega- and giga- denote powers of 1000, is also used for data transmission rates and DVD capacities. However, the convention is different from that used by manufacturers of memory (RAM, ROM) and CDs, where prefixes like kilo- and mega- mean powers of 1024.
When the unit prefixes like kilo- denote powers of 1024 in the measure of memory capacities, the 1024ⁿ progression (for n = 1, 2, ...) is as follows:^[30]

kilo = 2¹⁰ = 1024¹ = 1024,
mega = 2²⁰ = 1024² = 1,048,576,
giga = 2³⁰ = 1024³ = 1,073,741,824,
tera = 2⁴⁰ = 1024⁴ = 1,099,511,627,776,

and so forth.
The practice of using prefixes assigned to powers of 1000 within the hard drive and computer industries dates back to the early days of computing.^[32] By the 1970s million, mega and M were consistently being used in the powers of 1000 sense to describe HDD capacity.^[33]^[34]^[35] As HDD sizes grew the industry adopted the prefixes “G” for giga and “T” for tera denoting 1,000,000,000 and 1,000,000,000,000 bytes of HDD capacity respectively.
Likewise, the practice of using prefixes assigned to powers of 1024 within the computer industry also traces its roots to the early days of computing^[36] By the early 1970s using the prefix “K” in a powers of 1024 sense to describe memory was common within the industry.^[37]^[38] As memory sizes grew the industry adopted the prefixes “M” for mega and “G” for giga denoting 1,048,576 and 1,073,741,824 bytes of memory respectively.
Computers do not internally represent HDD or memory capacity in powers of 1024; reporting it in this manner is just a convention.^[39] Creating confusion, operating systems report HDD capacity in different ways. Most operating systems, including the Microsoft Windows operating systems use the powers of 1024 convention when reporting HDD capacity, thus an HDD offered by its manufacturer as a 1 TB drive is reported by these OSes as a 931 GB HDD. Apple's current OSes, beginning with Mac OS X 10.6 (“Snow Leopard”), use powers of 1000 when reporting HDD capacity, thereby avoiding any discrepancy between what it reports and what the manufacturer advertises.
In the case of “mega-,” there is a nearly 5% difference between the powers of 1000 definition and the powers of 1024 definition. Furthermore, the difference is compounded by 2.4% with each incrementally larger prefix (gigabyte, terabyte, etc.) The discrepancy between the two conventions for measuring capacity was the subject of several class action suits against HDD manufacturers. The plaintiffs argued that the use of decimal measurements effectively misled consumers^[40]^[41] 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.^[42]
In December 1998, an international standards organization attempted to address these dual definitions of the conventional prefixes by proposing unique binary prefixes and prefix symbols to denote multiples of 1024, such as “mebibyte (MiB)”, which exclusively denotes 2²⁰ or 1,048,576 bytes.^[43] In the over‑13 years that have since elapsed, the proposal has seen little adoption by the computer industry and the conventionally prefixed forms of “byte” continue to denote slightly different values depending on context.^[44]^[45]

[edit] HDD formatting

Main article: Disk formatting

The presentation of an HDD to its host is determined by its controller. This may differ substantially from the drive's native interface particularly in mainframes or servers.
Modern HDDs, such as SAS^[46] and SATA^[47] drives, appear at their interfaces as a contiguous set of logical blocks; typically 512 bytes long but the industry is in the process of changing to 4,096 byte logical blocks; see Advanced Format.^[48]
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.^[49]
High level formatting then writes the file system structures into selected logical blocks to make the remaining logical blocks available to the host OS and its applications.^[50] The operating system file system uses some of the disk space to organize files on the disk, recording their file names and the sequence of disk areas that represent the file. Examples of data structures stored on disk to retrieve files include the MS DOS file allocation table (FAT) and UNIX inodes, as well as other operating system data structures. As a consequence not all the space on a hard drive is available for user files. This file system overhead is usually less than 1% on drives larger than 100 MB.

[edit] Redundancy

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 for spares, thereby reducing capacity available to end users.
In some systems, there may be hidden partitions used for system recovery that reduce the capacity available to the end user.
For RAID drives, data integrity and fault-tolerance requirements also reduce the realized capacity. For example, a RAID1 drive will be about half the total capacity as a result of data mirroring. For RAID5 drives with x drives you would lose 1/x of your space to parity. RAID drives are multiple drives that appear to be one drive to the user, but provides some fault-tolerance. Most RAID vendors use some form of checksums to improve data integrity at the block level. For many vendors, this involves using HDDs with sectors of 520 bytes per sector to contain 512 bytes of user data and 8 checksum bytes or using separate 512 byte sectors for the checksum data.^[51]

[edit] HDD parameters to calculate capacity

PC hard disk drive capacity (in GB) over time. The vertical axis is logarithmic, so the fit line corresponds to exponential growth.

Because modern disk drives appear to their interface as a contiguous set of logical blocks their gross capacity can be calculated by multiplying the number of blocks by the size of the block. This information is available from the manufacturers specification and from the drive itself through use of special utilities invoking low level commands^[46]^[47]
The gross capacity of older HDDs can be calculated by multiplying for each zone of the drive the number of cylinders by the number of heads by the number of sectors/zone by the number of bytes/sector (most commonly 512) and then summing the totals for all zones. Some modern ATA drives will also report cylinder, head, sector (C/H/S) values to the CPU but they are no longer actual physical parameters since the reported numbers are constrained by historic operating-system interfaces.
The old C/H/S scheme has been replaced by logical block addressing. In some cases, to try to "force-fit" the C/H/S scheme to large-capacity drives, the number of heads was given as 64, although no modern drive has anywhere near 32 platters.

[edit] Form factors

5¼″ full height 110 MB HDD
2½″ (8.5 mm) 6495 MB HDD

2.5" SATA HDD from a Sony VAIO laptop

Six hard drives with 8″, 5.25″, 3.5″, 2.5″, 1.8″, and 1″ hard disks with a ruler to show the length of platters and read-write heads.

Mainframe and minicomputer hard disks were of widely varying dimensions, typically in free standing cabinets the size of washing machines (e.g. HP 7935 and DEC RP06 Disk Drives) or designed so that dimensions enabled placement in a 19" rack (e.g. Diablo Model 31). In 1962, IBM introduced its model 1311 disk, which used 14 inch (nominal size) platters. This became a standard size for mainframe and minicomputer drives for many years,^[52] but such large platters were never used with microprocessor-based systems.
With increasing sales of microcomputers having built in floppy-disk drives (FDDs), HDDs that would fit to the FDD mountings became desirable, and this led to the evolution of the market towards drives with certain Form factors, initially derived from the sizes of 8-inch, 5.25-inch, and 3.5-inch floppy disk drives. Smaller sizes than 3.5 inches have emerged as popular in the marketplace and/or been decided by various industry groups.

8 inch: 9.5 in × 4.624 in × 14.25 in (241.3 mm × 117.5 mm × 362 mm)
In 1979, Shugart Associates' SA1000 was the first form factor compatible HDD, having the same dimensions and a compatible interface to the 8″ FDD.
5.25 inch: 5.75 in × 3.25 in × 8 in (146.1 mm × 82.55 mm × 203 mm)
This smaller form factor, first used in an HDD by Seagate in 1980,^[53] was the same size as full-height 5¹⁄₄-inch-diameter (130 mm) FDD, 3.25-inches high. This is twice as high as "half height"; i.e., 1.63 in (41.4 mm). Most desktop models of drives for optical 120 mm disks (DVD, CD) use the half height 5¼″ dimension, but it fell out of fashion for HDDs. The Quantum Bigfoot HDD was the last to use it in the late 1990s, with "low-profile" (≈25 mm) and "ultra-low-profile" (≈20 mm) high versions.
3.5 inch: 4 in × 1 in × 5.75 in (101.6 mm × 25.4 mm × 146 mm) = 376.77344 cm³
This smaller form factor is similar to that used in an HDD by Rodime in 1983,^[54] which was the same size as the "half height" 3½″ FDD, i.e., 1.63 inches high. Today, the 1-inch high ("slimline" or "low-profile") version of this form factor is the most popular form used in most desktops.
2.5 inch: 2.75 in × 0.275–0.59 in × 3.945 in (69.85 mm × 7–15 mm × 100 mm) = 48.895–104.775 cm³
This smaller form factor was introduced by PrairieTek in 1988;^[55] there is no corresponding FDD. It is widely used today for solid-state drives and for hard disk drives in mobile devices (laptops, music players, etc.) and as of 2008 replacing 3.5 inch enterprise-class drives.^[56] It is also used in the Playstation 3^[57] and Xbox 360^{[citation needed]} video game consoles. Today, the dominant height of this form factor is 9.5 mm for laptop drives (usually having two platters inside), but higher capacity drives have a height of 12.5 mm (usually having three platters). Enterprise-class drives can have a height up to 15 mm.^[58] Seagate released a 7mm drive aimed at entry level laptops and high end netbooks in December 2009.^[59]
1.8 inch: 54 mm × 8 mm × 71 mm = 30.672 cm³
This form factor, originally introduced by Integral Peripherals in 1993, has evolved into the ATA-7 LIF with dimensions as stated. It was increasingly used in digital audio players and subnotebooks, but is rarely used today. An original variant exists for 2–5GB sized HDDs that fit directly into a PC card expansion slot. These became popular for their use in iPods and other HDD based MP3 players.
1 inch: 42.8 mm × 5 mm × 36.4 mm
This form factor was introduced in 1999 as IBM's Microdrive to fit inside a CF Type II slot. Samsung calls the same form factor "1.3 inch" drive in its product literature.^[60]
0.85 inch: 24 mm × 5 mm × 32 mm
Toshiba announced this form factor in January 2004^[61] for use in mobile phones and similar applications, including SD/MMC slot compatible HDDs optimized for video storage on 4G handsets. Toshiba currently sells a 4 GB (MK4001MTD) and 8 GB (MK8003MTD) version [1]^{[dead link]} and holds the Guinness World Record for the smallest hard disk drive.^[62]

3.5-inch and 2.5-inch hard disks currently dominate the market.
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,^[63]^[64] which is slightly more stable and resistant to damage from impact and/or dropping.
The inch-based nickname of all these form factors usually do not indicate any actual product dimension (which are specified in millimeters for more recent form factors), but just roughly indicate a size relative to disk diameters, in the interest of historic continuity.

[edit] Current hard disk form factors

Form factor	Width (mm)	Height (mm)	Largest capacity	Platters (Max)
3.5″	102	19 or 25.4	4 TB^[65]^[66]^[67]^[68] (2011)	5
2.5″	69.9	7,^[69] 9.5,^[70] 11.5,^[71] or 15	1.5 TB^[65]^[72]^[73]^[74] (2010)	4
1.8″	54	5 or 8	320 GB^[75] (2009)	3

[edit] Obsolete hard disk form factors

Form factor	Width (mm)	Largest capacity	Platters (Max)
5.25″ FH	146	47 GB^[76] (1998)	14
5.25″ HH	146	19.3 GB^[77] (1998)	4^[78]
1.3″	43	40 GB^[79] (2007)	1
1″ (CFII/ZIF/IDE-Flex)	42	20 GB (2006)	1
0.85″	24	8 GB^[80]^[81] (2004)	1