Days gone by and days yet to come are often defined by what is and what should never be.
[size=8pt]Yes, Robin has his Led Zeppelin records going in the Computer Centre last night whilst we were trying to hack up the magtape machine.
Turns out the problem, which I really should have suspected in the first place when everything first stopped working and started behaving in the most bizarre fashion imaginable, was a +2VDC ripple on the +5VDC rail in the machine's power supply. See, the expander module has two +5V rails which are stout enough to supply the current load, so I managed to get into it and switch the plugs around so the good one is runing the whole thing. At some point I may get in there and just pull out the whole expander and fix the bad power supply. For the moment everything appears to be working, and I have turned the backup notifications back on.
*sigh* And why it is always the power supply units? Why not an analogue board, or something interesting for once?