Hardware Reference
In-Depth Information
Figure 9.2. The tracks and sectors on a disk.
HDDs usually have multiple disks, called platters , that are stacked on top of each other and spin in
unison, each with two sides on which the drive stores data. Most drives have two or three platters,
resulting in four or six sides, but some PC hard disks have had up to 12 platters and 24 sides with 24
heads to read them (Seagate Barracuda 180). The identically aligned tracks on each side of every
platter together make up a cylinder (see Figure 9.3 ) . An HDD usually has one head per platter side,
with all the heads mounted on a common carrier device or rack. The heads move radially across the
disk in unison; they can't move independently because they are mounted on the same carrier or rack,
called an actuator .
Figure 9.3. Hard disk cylinders.
Originally, most hard disks spun at 3,600 rpm—approximately 10 times faster than a floppy disk
drive. For many years, 3,600 rpm was pretty much a constant among hard drives. Now, however,
most drives spin much faster. Although speeds can vary, modern drives typically spin the platters at
either 4,200 rpm, 5,400 rpm, 7,200 rpm, 10,000 rpm, or 15,000 rpm. Most standard-issue drives
 
 
 
 
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