If you change the mount letter within Disk Management it should stick to that drive when it is plugged into that machine.

The quick way to get into the Disk Management is to Right-click on My Computer then select Manage.  Once the window comes up, you should see Disk Management listed on the left-hand side.

Once you are in Disk Management, you should see a listing of all drives that the system is seeing.  You can then right-click on the drive in question and change the Drive-Letter.

Pretty easy, no? 8)

Hope this helps.