Microsoft Office Tutorials and References
In Depth Information
Hard Disks
Hard Disks
A hard disk is a storage device that contains one or more inflexible, circular platters that use
magnetic particles to store data, instructions, and information. The system unit on most personal
computers contains at least one hard disk, sometimes called an internal hard disk because it is not
portable. Users store documents, spread-
sheets, presentations, databases, e-mail
messages, Web pages, digital photos,
music, videos, and software on hard disks.
Hard disks store data and instructions in
tracks and sectors on a platter (Figure 17).
A track is a narrow recording band that
forms a full circle on the surface of the
disk. The disk’s storage locations consist
of pie-shaped sections, which break the
tracks into small arcs called sectors . On
a hard disk, a sector typically stores up to
512 bytes of data. Storage capacities of
internal hard disks for personal computers
range from 160 GB to more than 2 TB.
On desktop computers, platters most
often have a size of approximately 3.5 inches
in diameter. On notebook computers and
mobile devices, the diameter is 2.5 inches
or less. A typical hard disk has multiple
platters stacked on top of one another.
Each platter has two read/write heads, one
for each side. The hard disk has arms that
move the read/write heads to the proper
location on the platter (Figure 18). The
hard disk platters spin at a high rate of
speed, typically 5,400 to 15,000 revolutions
per minute. On today’s computers, the plat-
ters typically stop spinning or slow down
after a specified time to save power.
When reading or writing, the read/
write heads on a hard disk do not actually
touch the surface of the disk. The distance
between the read/write heads and the
platters is about two millionths of one
inch. This close clearance means that
dirt, hair, dust, smoke, or other particles
could cause the hard disk to have a head
crash , when a read/write head touches
a platter, usually resulting in loss of data
or sometimes the entire disk. Although
current hard disks are sealed tightly to
keep out contaminants, head crashes do
occur occasionally. Thus, it is crucial that
you back up your hard disk regularly.
A backup is a duplicate of a file, program,
or disk placed on a separate storage
medium that you can use in case the
original is lost, damaged, or destroyed.
3
sectors
track
1
18
2
17
16
4
15
14
5
13
6
12
7
11
8
10
9
Figure 17 Tracks form circles on the surface of a
hard disk platter. The disk’s storage locations are divided
into pie-shaped sections, which break the tracks into
small arcs called sectors.
How a Hard Disk Works
Step 1
The circuit board controls
the movement of the head
actuator and a small motor.
Step 2
A small motor spins the platters while
the computer is running.
Step 3
When software requests a
disk access, the read/write
heads determine the current
or new location of the data.
Step 4
The head actuator positions the
read/write head arms over the
correct location on the platters
to read or write data.
Figure 18
This figure shows how a hard disk works.
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