As I said in a private reply to Mark, this was indeed it - by getting into HiJack real early, just as the display lights up the first time, I could get to the HiJack functions.
Vitals showed 0+18GB and 1/2 showed 1 disk only. Changing the 1/2 option didn't save that boot - still didn't find the disk - but subsequent reboots have all been successful.

The fix you describe definately sounds like it would solve any future cases of this bit flipping (one wonders what caused it to flip though...).

Again, thanks Mark!

/Michael