How to force a drive letter for a USB drive
Solution 1:
This tool is an option:
http://www.uwe-sieber.de/usbdlm_e.html
Solution 2:
If you do that it is relatively permanent. I say relatively because if another drive takes that letter first then obviously it won't. The trick is to pick a letter further up the alphabet, far enough that nothing should take it first. I usually pick letters above J for just that reason.