The How-To Geek as a great article on how to resolve removable drive conflicts on your system.
Have you ever plugged in a USB drive or any external device with a hard drive and wondered why you cannot see it in My Computer?
More than likely the cause is that Windows renamed the drive to a letter that is already in use. This will happen if you have several card readers, thumb drives or external hard drives attached. It will also happen if you are on a network and have mapped drives.
I had this very same issue on my lap top. Nice step-wise procedure with lots of screen shots.