(SOLVED) Connection to external drive suddenly lost.

[besonian]

ThirdOfSix - excellent advice to mark the cables. I'm always promising myself to do that, along with marking the electric plugs - but somehow I never get around to it.  As to the USB cable I was using when the fault occurred, I've no idea how long I've had it. However, as it's marked 'USB SHIELDED HIGH SPEED CABLE 2.0' I guess it must be USB 2.0. Even so, that cable itself could be faulty or, as OP pointed out, the contacts on it may be dirty. But I'll come back to this later in the week anyway when I've had occasion to do more backups. Thank you for help. (Exits stage right to find cable-marking felt pen) 

[besonian]

OK - I think this is the end of this matter. I hope anyway. And after all my testing and re-testing of the drive and the file-system etc., it really looks like the problem - on both machines - was the USB cable I was using. Though I've no idea exactly how old they were, the certainly go back a bit. So now I'm using a new one with that drive on both machines and things are working - so far at least -  exactly as they're suppose to. Thank you guys, as always, for all the input. I think it's fair now to put a large 'Solved' notice on this thread.