I had same problem.
And I have an solution for you... The problem is that when you plug in your HD first time, Windows 7 install your HD and associate it to only this USB port (let it be USB port "4") than if you wanna to use it (HD) again, you need to plug it, in the same USB port (USB port "4") where it was installed first time. It is Stupid!!!! But that's how it worked with my HD.
Just try all USB ports that you have on your PC.
Good luck.
Hope it was helpful. :D
PS: the same problem is with all divices you plug in to your pc.