Thanks Ant. As far as I can remember, I don't have USB debugging enabled, though it's perfectly possible. You have to enable it to install dodgy apks, don't you? So yes, it's possible.

The warranty replacement for the HDD is a good thought. Still got a month and a half to go. Though as you know, the siren call of the Adat M2 2TB tbh is tempting me. Perhaps it's God's way of prodding me.

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?