A condition that can prematurely cause failure in hard disk drives.

After many billions of revolutions (a 7200rpm drive will rotate 4320000 times in a 10 hour session) microscopic pieces of the alloy that makes up the spindle will shear off and cause friction in the mechanism. When enough of these particles build up, the drive can have trouble coming up to speed. It may appear to be dead.

The easiest fix is to thump* the device on it's flat side with the heel of your palm and replace. You might have to do this every few months, but it can increase the life of the drive for quite some time.

*ensure that the drive is powered off and spun down, obviously.