Got USB Debugging enabled on the tablet?
I find without that enabled previously you're often stuck to be able to do anything over USB, which is obviously very frustrating. Much like the auto-charge setting when on USB and not File Transfer. Meant I couldn't backup my mum's phone when the screen failed.

Your HDD one is crazy though!
Not seen in Device Manager either?

I'd recommend checking SMART status, backing up the data and getting it warranty replaced. Maybe try and linux live USB and see if that kicks up any fuss for the HDD.

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?