Drive heads get dirty in two ways: particles on the disk surface like dust
and cigarette smoke can stick to the head and through normal use of the disk's
magnetic coating which rubs off on the head. These coatings vary in quality.
Disks that have been in humid environments can clog the drive heads after just
50 or 60 disks.
When the heads gets dirty, they no longer make good contact with the disk and
the dirt particles diffuse the reading and writing process. This produces
erratic results: some disks work fine while others are inexplicably rejected.
