curmudgeon,
I know this is a bit of an unsatisfactory fix, but if its the same problem as mine, this could help if you need to get the machine booted up quickly.
Not as unsatisfactory as you may think... in the days when a single computers came in multiple boxes big enough to
climb into (that dates me
) part of the quaterly maintenance cycle involved
replacing all the cables between the drive enclosure and the CPU enclosure
Themal creep of the cable connectors is not that uncommon...