External Disc Drive

I have an old hard drive which I'm trying to use on my Windows 7 system. It did work and does still work on an XP system but now for reasons which I can't understand no longer works on Windows 7. The disc appears in Device Manager alongside my installed 'C' drive but it doesn't appear on Computer Manager display. Can anyone shed any light on why this might be? I haven't changed any hardware recently and software changes are mostly Windows Updates.

What type of hard drive is it? USB, fire wire or E-sata.

Did it have a drive letter before? It may be as simple as assigning a drive letter to it.
Go to Device manager and right click it. If enable is an option, click it.

It's USB and I have looked at it on device manager but it was already enabld. I tried disabling, and then re-enabling, but there still wasn't an option to assign a drive letter. However, I did find that it was on 'Disc Management' and was able to assign a drive letter that way which did work. I used the disc successfully and will see if it is still there when I next tried connecting it tomorrow

Once you assigned it a drive letter through Disc Management it should be recognized with no problems now.