Have you given this drive a dedicated drive letter?
Open RUN, type in "diskmgmt.msc'. When it opens, find the drive and give it a dedicated drive letter.
It may be that the drive has become corrupted - type in chkdsk in the search field under all programmes (click on the start button first).
You should able to find out what it says about the drive. There may be some advice to follow. Does it say that the file system is RAW? If so then for some reason it has been corrupted - could it have been disconnected from pc without using the remove safely hardware facility for example? In some instances Windows 7 doesn't like external hard drives being left plugged in while you are shutting down or logging on.
Have you tried this with another pc?