The sata cable had a split - I tested it on the other drive & it worked, but I replaced it anyway & tried the good one on the missing drive - still no dice, so I can eliminate that as the problem. The drives have an extra case fan blowing on them, added by me, since it's a half height htpc case - it gets warm. Even so, the drives run at around 45-47 degrees - warm but not excessive & they're both new-ish.
I've rebuilt Vista on the original drive as I said and it's running tickety boo...for now. I'm supposing that something went skibaa in the bootloader interface but it's really not anything I know about - yes, I tend to poo poo anything from the Linux brigade - they usually come armed with an agenda rather than a desire to help - I'm after someone well acquainted with Bios's & HDD's.