On Aug 28, 2007, at 1:49 PM, Martin McCormick wrote:
I think some drives exercise the head rack during lulls of activity to keep it from stiffening up.
Some older drives also periodically move the head around to do thermal recalibration. As the drive warms up the platters expand and the head positioning logic has to compensate. Modern drives do this automatically by using embedded servo tracks, but older ones had to recalibrate every few minutes. It was a big headache for some A/V applications, because it interrupted the data stream.