what is hard disk?

10:38 AM in computer hardware

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.