From ECDL.web

A hard disk drive with protective cover removed.

A hard disk drive (HDD) s a non-volatile, random access device for storing digital data. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the platters. Introduced by IBM in 1956, hard disk drives have fallen in cost and physical size over the years while dramatically increasing in capacity. Hard disk drives have been the dominant device for secondary storage of data in general purpose computers since the early 1960s. Since then HDDs have changed in many ways, a few highlights include:

• Capacity increasing from 3.75 megabytes to greater than 1 terabyte,
• Size decreasing from 87.9 cubic feet (a double wide refrigerator) to 0.002 cubic feet (2½-inch form factor, a pack of cards),
• Price decreasing from about $15,000 per megabyte to less than $0.0001 per megabyte,
• Average access time decreasing from greater than 0.1 second to a few thousandths of a second,
• Market application expanding from general purpose computers to most computing applications including consumer applications.

[edit] Form factors

Six hard drives with 8″, 5.25″, 3.5″, 2.5″, 1.8″, and 1″ disks, partially disassembled to show platters and read-write heads, with a ruler showing inches.

Mainframe and minicomputer hard disks were of widely varying dimensions, typically in free standing cabinets the size of washing machines or designed so that dimensions enabled placement in a 19" rack. 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, 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. 3.5-inch and 2.5-inch hard disks currently dominate the market.

• 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, was the same size as full-height 51⁄4-inch-diameter floppy disk drive.
• 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, first used in an HDD by Rodime in 1983, was the same size as the "half height" 3½″ FDD, i.e., 1.63 inches high. Today it has been largely superseded by 1-inch high "slimline" or "low-profile" versions of this form factor which is used by most desktop HDDs.
• 2.5 inch: This smaller form factor was introduced by PrairieTek in 1988. It is widely used today for hard-disk drives in mobile devices (laptops, music players, etc.) and as of 2008 replacing 3.5 inch enterprise-class drives. It is also used in the Playstation 3 and Xbox 360 video game consoles.
• 1.8 inch: This form factor, originally introduced by Integral Peripherals in 1993, was increasingly used in digital audio players and subnotebooks, but is rarely used today.
• 1 inch: This form factor was introduced in 1999 as IBM's Microdrive to fit inside a Compact Flash Type II slot to be used in digital cameras.
• 0.85 inch: Toshiba announced this form factor in January 2004 for use in mobile phones and similar applications. Toshiba currently sells a 4 GB and 8 GB version and holds the Guinness World Record for the smallest hard disk drive.

[edit] Access and interfaces

Hard disk drives are accessed over one of a number of bus types, including parallel ATA (P-ATA, also called IDE or EIDE), Serial ATA (SATA), SCSI, Serial Attached SCSI (SAS), and Fibre Channel. Bridge circuitry is sometimes used to connect hard disk drives to buses that they cannot communicate with natively, such as IEEE 1394, USB and SCSI.

Modern hard drives present a consistent interface to the rest of the computer, no matter what data encoding scheme is used internally. The electronics inside the hard drive takes the raw analog voltages from the read head and sends that data out the standard interface. It also watches the error rate detected by error detection and correction, and performs bad sector remapping, slef-monitoring and other internal tasks.

[edit] Disk interface families used in personal computers

Inner view of a 1998 Seagate hard disk drive which used Parallel ATA interface

• Integrated Drive Electronics (IDE), later renamed to ATA, with the alias P-ATA ("parallel ATA") retroactively added upon introduction of the new variant Serial ATA. The original name reflected the innovative integration of HDD controller with HDD itself, which was not found in earlier disks. Moving the HDD controller from the interface card to the disk drive helped to standardize interfaces, and to reduce the cost and complexity.
• EIDE was an unofficial update by Western Digital to the original IDE standard, with the key improvement being the use of direct memory access to transfer data between the disk and the computer without the involvement of the CPU, an improvement later adopted by the official ATA standards.
• Small Computer System Interface (SCSI), originally named SASI for Shugart Associates System Interface. SCSI disks were standard on servers, workstations, Commodore Amiga, and Apple Macintosh computers through the mid-1990s, by which time most models had been transitioned to IDE (and later, SATA) family disks. Only in 2005 did the capacity of SCSI disks fall behind IDE disk technology, though the highest-performance disks are still available in SCSI and Fibre Channel only.
• Serial ATA (SATA) is a computer bus interface for connecting mass storage devices such as hard disk drives and optical drives. It has several advantages over the older parallel ATA (PATA) interface: reduced cable-bulk and cost, native hot swapping and faster data transfer through higher signalling rates.
• Fibre Channel is a gigabit-speed network technology primarily used for storage networking. Fibre Channel was primarily used in the supercomputer field, but now has become the standard connection type in enterprise storage. Despite its name, Fibre Channel signaling can run on both twisted pair copper wire and fiber-optic cables.
• Serial Attached SCSI (SAS) is a new generation serial communication protocol for devices designed to allow for much higher speed data transfers and is compatible with SATA.

[edit] Market segments

As of July 2010, the highest capacity consumer HDDs are 3 TB. Desktop HDDs typically store between 120 GB and 2 TB and rotate at 5,400 to 10,000 rpm, and have a media transfer rate of 0.5 Gbit/s or higher.

Enterprise HDDs are typically used with multiple-user computers running enterprise software. The fastest enterprise HDDs spin at 10,000 or 15,000 rpm, and can achieve sequential media transfer speeds above 1.6 Gbit/s and a sustained transfer rate up to 1 Gbit/s. Enterprise drives commonly operate continuously in demanding environments while delivering the highest possible performance without sacrificing reliability. Maximum capacity is not the primary goal, and as a result the drives are often offered in capacities that are relatively low in relation to their cost.

Mobile HDDs or laptop HDDs, smaller than their desktop and enterprise counterparts, tend to be slower and have lower capacity. A typical mobile HDD spins at either 4200 rpm, 5200 rpm, 5400 rpm, or 7200 rpm, with 5400 rpm being the most prominent. Because of smaller platters, mobile HDDs generally have lower capacity than their greater desktop counterparts.