Hard Disk Storage - Upgrading and Repairing PCs - page 561

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 identic-
ally 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 com-
mon 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 ac-
tuator .
Figure 9.3 Hard disk cylinders.