Modern (as of 1998) hard drives may contain as many as 10000 tracks per inch. The disk heads can be positioned to about 6/100s of the track width, however a variety of factors may cause the data track to be slightly off center at any given time. Further, the heads are very light and may vibrate slightly in sympathy with vibrations in the PC -- as may occur when another drive seeks to a new track. The disks are able to monitor their centering in the track using servo data written permanently on the disk surface and will automatically shut off reads or writes if the head moves more than about 10/100s to 15/100s of the track width off of dead center. This prevents accidentally reading from or writing data to adjacent tracks.
Temporary shutdowns during read/write are not regarded as hard errors. The drive merely waits a rotation then tries again.
Typical PC Disk mounting brackets are not very sturdy and the situation commonly encountered during disk testing of an unmounted drive sitting on top of a PC bay is even more likely to vibrate the drive. If the vibrations are large enough, they can introduce transient shutdowns of reading and/or writing. The resulting missed revolutions can seriously affect measured disk performance. It is estimated that overall performance of a disk drive in a typical PC may easily be 20% below that measured in a laboratory with the disk firmly attached to a solid mounting. It has been reported that some popular and well-known cases have very poor vibration performance. An additional factor is that servers containing a number of drives may exhibit much more disk vibration than workstations which typically have only one or two drives and a fan causing vibrations.
It should also be mentioned that soft mountings -- eg foam pads -- do, as one would expect, minimize the affect of external vibrations. But they also maximize any self induced vibrations. For best disk performance, solid, secure, mountings to heavy structural members are recommended.
Return To Index Copyright 1994-2008 by Donald Kenney.