I wonder if the SATA cable plug is wobbly on either end (or both) ?

I wondered the same. I've given the connecting plugs (power as well) a bit of a push. I've rebooted a couple of times, without problems. We'll see.

got a different sata port on the motherboard you could try?

Another good idea, Milky Bar Kid. So far wiggling my cables seems to have done the trick, but that's a good (cheap) second move if it's not sorted.

Of course, that would point to a possible MB failure.  :-S

How long you had it? Self-build? Any lightning strikes / power surges?