A hard disk is part of a unit, often
called a "disk drive," "hard drive," or "hard disk
drive," that stores and provides relatively quick access to large amounts
of data on an electromagnetically charged surface or set of surfaces. Today's
computers typically come with a hard disk that contains several billion bytes
(gigabytes) of storage.
A hard disk is really a set of
stacked "disks," each of which, like phonograph records, has data
recorded electromagnetically in concentric circles or "tracks" on the
disk. A "head" (something like a phonograph arm but in a relatively
fixed position) records (writes) or reads the information on the tracks. Two
heads, one on each side of a disk, read or write the data as the disk spins.
Each read or write operation requires that data be located, which is an
operation called a "seek." (Data already in a disk cache,
however, will be located more quickly.)
A hard disk/drive unit comes with a
set rotation speed varying from 4500 to 7200 rpm. Disk access time is measured
in milliseconds. Although the physical location can be identified with
cylinder, track, and sector locations, these are actually mapped to a logical
block address (LBA) that works with the larger address range on
today's hard disks.
Related glossary terms: magnetoresistive head technology, yottabyte, serverless backup, byte, partition, InfiniBand, failover, RAMAC (random access method of accounting and control),
Fibre Channel, continuous data protection (storage convergence)
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