A ( ; also , , or ) is a device for storing and retrieving digital information, primarily computer data. It consists of one or more rigid (hence "hard") rapidly rotating discs (often referred to as platters), coated with magnetic material and with magnetic heads arranged to write data to the surfaces and read it from them. Hard drives are classified as non-volatile, random a ccess, digital, magnetic, data storage devices. Introduced by IBM in 1956, hard disk drives have decreased in cost and physical size over the years while dramatically increasing in capacity and speed. Hard disk drives have been the dominant device for secondary storage of data in general purpose computers since the early 1960s. They have maintained this po sition because advances in their recording capacity, cost, reliability, a nd speed have kept pace with the requirements for secondary storage.
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and retrieving digital information, primarily computer data. It consists of one or more rigid
(hence "hard") rapidly rotating discs (often referred to as platters), coated with magnetic
material and with magnetic heads arranged to write data to the surfaces and read it from
them.
Hard drives are classified as non-volatile, random access, digital, magnetic, data storage
devices. Introduced by IBM in 1956, hard disk drives have decreased in cost and physicalsize over the years while dramatically increasing in capacity and speed.
Hard disk drives have been the dominant device for secondary storage of data in general
purpose computers since the early 1960s. They have maintained this position because
advances in their recording capacity, cost, reliability, and speed have kept pace with the
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.
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 then writes the file system structures into selected logical blocks to
make the remaining logical blocks available to the host OS and its applications. 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. Examplesof 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.
Most hard disk drives today support some form of power management which uses a
number of specific power modes that save energy by reducing performance. When
implemented an HDD will change between a full power mode to one or more power saving
modes as a function of drive usage. Recovery from the deepest mode, typically called Sleep,
may take as long as several seconds.
Shock resistance is especially important for mobile devices. Some laptops now include
active hard drive protection that parks the disk heads if the machine is dropped, hopefully
before impact, to offer the greatest possible chance of survival in such an event. Maximum
shock tolerance to date is 350 g for operating and 1000 g for non-operating
Power consumption has become increasingly important, not only in mobile devices such as
laptops but also in server and desktop markets. Increasing data center machine density has
led to problems delivering sufficient power to devices (especially for spin up), and getting
rid of the waste heat subsequently produced, as well as environmental and electrical cost
concerns (see green computing). Heat dissipation is tied directly to power consumption,
and as drives age, disk failure rates increase at higher drive temperatures. Similar issues
exist for large companies with thousands of desktop PCs. Smaller form factor drives often
use less power than larger drives. One interesting development in this area is actively
controlling the seek speed so that the head arrives at its destination only just in time to
read the sector, rather than arriving as quickly as possible and then having to wait for the
sector to come around (i.e. the rotational latency). Many of the hard drive companies are
now producing Green Drives that require much less power and cooling. Many of these
Green Drives spin slower (<5400 rpm compared to 7200, 10,000 or 15,000 rpm) thereby
generating less heat. Power consumption can also be reduced by parking the drive headswhen the disk is not in use reducing friction, adjusting spin speeds, and disabling internal
components when not in use.
On SCSI hard disk drives, the SCSI controller can directly control spin up and spin
Plug and play drive functionality offers system compatibility, and features large volume data
storage options, but maintains a portable design.
These drives with an ability to function and be removed simplistically, have had further
applications due their flexibility. These include:
Disk cloning
Data storage
Data recovery
Backup of files and information
Storing and running virtual machines
Scratch disk for video editing applications and video recording
Booting operating systems
External hard disk drives are available in two main sizes (physical size), 2.5" and 3.5".
Features such as biometric security or multiple interfaces are available at a higher cost
Hard drives may fail in a number of ways. Failure may be immediate and total, progressive,
or limited. Data may be totally destroyed, or partially or totally recoverable. Earlier drives
tended to develop bad sectors with use and wear, which could be "mapped out" so that they
did not affect operation; this was considered normal unless many bad sectors developed in a
short period. Later drives map out bad sectors automatically and invisibly to the user;S.M.A.R.T. information logs these problems. A drive with bad sectors may usually continue
to be used.
Other failures which may be either progressive or limited are usually considered to be a
reason to replace a drive; the value of data potentially at risk usually far outweighs the cost