Funnily enough, my PC had started to (from time to time) fail to recognise one or both of the HDDs on POST, or most often would recognise them but not boot up. Fiddling about with the cables and stuff a bit would often sort it out, along with maybe a bit of mucking about in the BIOS with boot priorities and stuff.

The other day it did it again, and I noticed in the BIOS all the dates were wrong and the clock was at 0:00. Swapped the buttone cell battery (for one i'd only just put in the kitchen scales) and it worked. Seems OK so far, let's hope it stays that way.

Oh, it's an ASUS mobo