Hard disk

A hard disk drive (HDD, or also hard drive or the now-obsolete usage hard file) is a non-volatile data storage device that stores data on a magnetic surface layered onto hard disk platters.
From the original use of a hard drive in a single computer, techniques for guarding against hard disk failure were developed such as the redundant array of independent disks (RAID). Hard disks are also found in network attached storage (NAS) devices, but for large volumes of data are most efficiently used in a storage area network (SAN). Applications for hard disk drives expanded to include video recorders, audio players, digital organizers, and digital cameras. In 2005 the first cellular telephones to include hard disk drives were introduced by Samsung and Nokia.

Capacity measurements

Hard drive manufacturers typically specify drive capacity using 'SI prefixes', that is, the SI definition of the prefixes "giga" and "mega." This is largely for historical reasons, since disk drive storage capacities exceeded millions of bytes long before there were standard 'binary prefixes' (even before there were the SI prefixes, 1960). The IEC only standardized 'binary prefixes' in 1999. As it turned out, many practitioners early on in the computer and semiconductor industries adopted the term kilobyte to describe 210 (1024) bytes because 1024 is "close enough" to the metric prefix kilo, which is defined as 103 or 1000. Sometimes this non-SI conforming usage include a qualifier such as '"1 kB = 1,024 Bytes"' but this qualifier was frequently omitted, particularly in marketing literature. This trend became habit and continued to be applied to the prefixes "mega," "giga," "tera," and even "Peta (prefix)."
Operating systems and their utilities, particularly visual operating systems such as Microsoft's various Windows operating systems frequently report capacity using binary prefixes which results in a discrepancy between the drive manufacturer's stated capacity and the system's reported capacity. Obviously the difference becomes much more noticeable in reported capacities in the multiple gigabyte range, and users will often notice that the volume capacity reported by their OS is significantly less than that advertised by the hard drive manufacturer. For example, Microsoft's Windows 2000 reports drive capacity both in decimal to 12 or more significant digits and with binary prefixes to 3 significant digits. Thus a disk drive specified by a drive manufacturer as a '30 GB' drive has its capacity reported by Windows 2000 both as '30,065,098,568 bytes' and '28.0 GB'. The drive manufacturer has used the SI definition of "giga," 109 and can be considered as an approximation of a gibibyte. Since utilities provided by the operating system probably define a gigabyte as 230, or 1073741824, bytes, the reported capacity of the drive will be closer to 28.0 GB, a difference of well over 7%. For this very reason, many utilities that report capacity have begun to use the aforementioned IEC standard binary prefixes (e.g. KiB, MiB, GiB) since their definitions are unambiguous.
Many people mistakenly attribute the discrepancy in reported and specified capacities to reserved space used for file system and partition accounting information. However, for large (several GiB) filesystems, this data rarely occupies more than a few MiB, and therefore cannot possibly account for the apparent "loss" of tens of GBs.

Drive families used in personal computers

Notable drive families include:

• MFM (Modified Frequency Modulation) drives required that the controller electronics be compatible with the drive electronics.
• RLL (Run Length Limited) drives were named after the modulation technique that made them an improvement on MFM. They required large cables between the controller in the PC and the hard drive, the drive did not have a controller, only a modulator/demodulator.
• ESDI (Enhanced Small Disk Interface) was an interface developed by Maxtor to allow faster communication between the PC and the disk than MFM or RLL.
• IDE (Integrated Drive Electronics) was later renamed to ATA, and then PATA.

The name comes from the way early families had the hard drive controller external to the drive. Moving the hard disk controller from the interface card to the drive helped to standardize interfaces, reducing cost and complexity.
The data cable was originally 40 conductor, but UDMA modes from the later drives requires using an 80 conductor cable (note that the 80 conductor cable still uses a 40 position connector.)
The interface changed from 40 pins to 39 pin. The missing pin acts as a key to prevent incorrect insertion of the connector, a common cause of drive and controller damage.

• SCSI (Small Computer System Interface) was an early competitor with ESDI, originally named SASI for Shugart Associates. SCSI drives were standard on servers, workstations, and Apple Macintosh computers through the mid-90s, by which time most models had been transitioned to IDE (and later, SATA) family drives. Only in 2005 did the capacity of SCSI drives fall behind IDE drive technology, though the highest-performance drives are still available in SCSI and Fibre Channel only. The length limitations of the data cable allows for external SCSI devices. Originally SCSI data cables used single ended data transmission, but server class SCSI could use differential transmission, and then Fibre Channel (FC) interface, and then more specifically the Fibre Channel Arbitrated Loop (FC-AL), connected SCSI hard drives using fibre optics. FC-AL is the cornerstone of storage area networks, although other protocols like iSCSI and ATA over Ethernet have been developed as well.
• SATA (Serial ATA). The SATA data cable has only one data pair for the differential transmission of data to the device, and one pair for receiving from the device. That requires that data be transmitted serially. The same differential transmission system is used in RS485, Appletalk,USB, Firewire,and differential SCSI. In 2005/2006 parlance, the 40 pin IDE/ATA is called "PATA" or parallel ATA, which means that there are 16 bits of data transferred in parallel at a time on the data cable.

EIDE was an unofficial update (by Western Digital) to the original IDE standard, with the key improvement being the use of DMA to transfer data between the drive and the computer, an improvement later adopted by the official ATA standards. DMA is used to transfer data without the CPU or program being responsible to transfer every word. That leaves the CPU/program/operating system to do other tasks while the data transfer occurs.