It seems like it might be an IDE drive that is about 7 years old.
I'd check all the connections, inside the third party case especially.
Try a different USB cable if possible.

Even remove it and hook it up directly to a MB port if possible.
If so, Be sure the BIOS is set to detect it properly.
(Auto has worked well for me with drives that size and age.)
That will remove as many variables as possible.
Be sure to mark/record how the drive is jumpered in the case
(for the "next one?/replacement")
and then jumper it properly when moved elsewhere.

If it still will not work try it on an IDE data cable by itself, jumpered
properly.  (I'm stressing the jumpers in all this as that is a big part
of getting an IDE drive to "blend in" correctly...)

If all the above fails, I'd still try it in a different MB, or on an
IDE-to-USB adapter cable. (I have all this stuff...)

I exhaust all options, but then there is the old saying.
It is not "if" a HD will fail, just "when"...

Good luck,  Rick Glazier
CC: OP, please reply ONLY on list.

From: "Ronald Smith"
>I have a usb hard drive for backup (WDC WD40 0BB-00DEA0 USB Device) which comes on when plugged in (blue light, vibrations), but
>does not show up in My Computer, despite appearing  in Device Manager and showing up as working and having drivers.

>This HD in its enclosure( Pluscom 3.5 IDE to USB 2.0) did show previously and was used to backup the C drive. But now it is absent
>from  My Computer in all three computers on which it has been tested.

>Why is it no longer recognized, and how can I restore it to use?

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