I used to have a problem of that sort with my laptop format external USB HDD. Turns out there wasn't enough power so while the controller was recognized by Windows, it failed to used it because the HDD failed to spin to operational speed. I bought another
one with dual USB cable (one for data and one for power) and never had that problem anymore.
Yeah, though they said external power may be needed, it's safe to say it is required.
I have an Adaptec External case that came with an external power supply. Specifically this one:
I wonder if something could be the matter with it?
Some times PnP assignes the drive a letter which is in used by a network drive mapping, so you need to go into disk managment and change the drive letter assignment to one that's not in use, Windows normally remembers the assignment
Every time the drive has come up it has come up with the same letter designation as before. So I know Windows can see and use it. I have used it before, but it seems to be a miracle to work again at times.