OS neuvádíš.
http://www.howtogeek.com/96298/assign-a-static-dri ve-letter-to-a-usb-drive-in-windows-7/
http://www.cnet.com/forums/discussions/permanently -assign-drive-letter-to-external-hd-how-to-303321/
Používám USB Safely Remove, umí toho mnohem víc.