Could you post your PC spec - you can use Belarc found here :
http://www.pcworld.com/downloads/fil...id,8078,00.asp
Could you also post the make and model number of your external USB drive.
Might be worth going into device manager and making sure there are no conflicts.
Also boot your machine into safe mode, go into device manager and delete the USB devices.(should be at the bottom of the list) and then restart your machine. It should re-install your USB. once installed (it might ask you to restart) try your hard drive again