If you turn the HD off, and with the computer running plug in the USB lead and power the HD up, what happens? Do you get activity on the task bar showing that a 'mass storage device' has been detected etc. ?
I can't remember exactly how you do this, you right click on the My Computer icon and goto Manage I think and then click on Disk Management but I can't recall what you do after that, there is some sort of option to enable the drive to be seen but I cannot recall where it is.
Hopefully someone else will give you the ending to what to do.
There will be an option in device manager to disable the drive, which would obviously need to be changed if so, but I think you would see a red cross against entry in device manager for it if this were so. Does the USB icon in the system tray report that it 'cannot find driver' for the device?
I have seen a similar problem before. There was a conflict with two drives trying to use the same drive letter. If you go into Disk Management and Right Click on the USB device you wil get an option to change the drive letter. Change it to a lettr which is not used by any other drive (including network drives).