Shows how little I know.

I had/have, as you can see, an external hard-drive.
It packed up some time ago: it befell thusly ...
It went from being there at boot up, to being there, apparently, just as and when it felt so inclined: thence to not there at all.
Eventually the "Power on" light vanished. After a deal of faffing I found another, compatible, power source - still no worky.
Tried the local Apple© resellers, who diagnosed death.

What I was unaware of was the fact that the case did anything other than hold/protect the drive, they pointed me in the direction of a Cyber Cafe, who diagnosed the fact that it might very well be only the fault of the "Caddy".
Well, ... YIPPEEEeeeeee!
A test proved this 2 B the case.
Result - up and running with no loss of data.