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.
http://www.justvb.net/it/