Spindle, platters, and heads: A disk is made up of several platters on
which data is stored. The platters are all mounted on a single spindle
and rotate as a unit. Data is stored on the platters in tracks. Each track
is a circle with the spindle as its center, and each track is at a different
radius from the center. A cylinder is all the tracks at a given radius on
all the platters. Data is stored in sectors, or blocks, within the track, and
tracks have different numbers of blocks based on how far from the center
they are. Tracks farther from the center are longer and therefore have
more blocks. The heads read and write the data on the disk by hovering
over the appropriate track. There is one head per platter, but they are
all mounted on the same robotic arm and move as a unit. Generally,
an entire track or an entire cylinder will be read at once, and the data
cached, as the time it takes to move the heads to the right place (seek
time) is longer than it takes for the disk to rotate 360 degrees.