"bitrot" on magnetic media: is there such a thing?

[John Cornelius]

Bits normally do not rot on magnetic media, at least not in the lifetime of a
winchester disk drive.  Bits have been known to spread and/or migrate on reels
of magnetic tape that have been stored for long periods of time (7-10 years)
but one would not expect this behaviour on a disk drive.

The most probable causes of 'bit rot' on a disk drive are:
1)	Worn or defective erase heads, might be caused by head crashes or
	chronic power off on the disk.  When you power down a winchester the
	heads crash and are subject to wear.

2)	Worn or defective write heads, they can spread the bits out resulting
	in lower signal/noise ratio.

3)	Defective media.  The retentivity of the media may not be good enough.
	The tendency of magnetic media is toward a uniform polarization.  The
	media is designed to have a half life in the tens of years and in some
	cases hundreds of years.  The drive itself should disintegrate before
	the half life is reached.  On the other hand, nobody's perfect and on
	occasion less than ideal media winds up in disk drives.

4)	Impurities in the HDA environment can make the heads less sensitive and
	less precise.  This is the infamous problem with the RA-81 where glue
	vaporized inside the HDA and began coating the heads and disks.  The
	usually untimely result is a catastrophic head crash but read errors
	often precede the crash so you have some warning.

5)	Defective head selection matrix resulting in small write currents on
	unselected heads during writing.  This will often be followed by a
	catastrophic failure and hard 'write-fault' or 'head select' errors.

The first two of these causes can be avoided by never turning your winchester
off.  The last three are harder to avoid but judicious selection of disk
vendors can be a help.  Lowest purchase price does not usually have anything to
do with lowest cost to own.

The wisdom of leaving your winchester running, even if the system it is
connected to is not running, cannot be too heavily stressed.  Winchesters are
designed for a continuous operating environment, not a sporadic one.  There is
a school of thought that being nice to your disk drive involves turning it off
when it is not in use.  I recognize that this thinking has some intuitive basis
but it is, alas, quite incorrect.

John Cornelius
aka jc at sdcsvax