Microsoft Office Tutorials and References
In Depth Information
COM 12
ESSENTIAL INTRODUCTION TO COMPUTERS
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H ARD D ISKS A hard disk , also called a hard disk drive, is a storage device that contains one or
more inflexible, circular platters that magnetically store data, instructions, and information. Home
users store documents, spreadsheets, presentations, databases, e-mail messages, Web pages,
digital photographs, music, videos, and software on hard disks. The data on hard disks is recorded
on a series of tracks located on one or more platters. The tracks are divided into sectors when the
disk is formatted. Figure 17 shows how a hard disk works. The hard disk platters spin at a high
rate of speed, typically 5,400 to 15,000 revolutions per minute. When reading data from the disk,
the read head senses the magnetic spots that are recorded on the disk along the various tracks
and transfers that data to memory. When writing, the data is transferred from memory and is
stored as magnetic spots on the tracks on the recording surface of one or more of the disk platters.
When reading or writing, the read/write heads on a hard disk drive 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, dust, smoke, or other particles could
cause a head crash , when a read/write head touches a platter, usually resulting in loss of data or
sometimes the entire drive. Although current hard disks are sealed tightly to keep out
contaminants, head crashes do occasionally occur. Thus, it is crucial that you back up your hard disk
regularly. A backup is a duplicate of a file, program, or disk that you can use in case the original is
lost, damaged, or destroyed.
The number of platters permanently mounted on the spindle of a hard disk varies. On most
drives, each surface of the platter can be used to store data. Thus, if a hard disk drive uses one
Hard Disks
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Hard Disks.
How a hard disk works.
FIGURE 17
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.
Step 1:
The circuit board controls
the movement of the head
actuator and a small motor.
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