all of the other read/write heads are over track 29 on each respective disk. Disk organiza-
tion and tracks are discussed later in the chapter. In most disk drive designs, only one
read/write head is active at a time.Floating Heads
The read/write heads float over the surface of the platters on a cushion of air pressure.
When the disk drive is off and the platters are not turning, the springs in the head arms
actually force the read/write heads onto the surface of the disk. But when the drive is
operating, the high-speed rotation of the disk platters creates air pressure that pushes the
read/write head away from the disk surface. The springs in the head arms provide resis-
tance so that the read/write head floats above the disk’s surface at a constant height,
which is around three to five microinches (millionths of an inch).
The ga pbetween the platter and the read/write head is so small that serious damage
canhappentotheread/writeheadifitbangsintoanyforeignobstacleonthedisk.Particles
like dust or smoke, which are like the Himalayas to the read/write head, can cause the
read/write head to crash on the disk’s surface. Smoke particles and the oil of a human
fingerprint are well over a thousand microinches in height. The HDA is a sealed unit and
it is very unlikely that this will happen, but this is a very good reason for you not to open
the HDA for any reason, unless you just happen to have a class-100 environment. Disk
drives are manufactured in this type of environment, which doesn’t allow more than 100
tiny foreign particles airborne in the facility. Just for reference, humans exhale more
than 500 such particles each time they breathe.
Whenthepoweristurnedoff,thediskstopsspinning.Thiseliminatestheairpressure
cushion on which the read/write head was riding. Although this sounds like a disk head
crash in the making, disk drives have alanding zone, beyond the inside edge of the recording(^176) PC Hardware: A Beginner’s Guide
Figure 9-4. Each platter has a read/write head for each of its sides