Connect your USB external disk.
Mine is a Maxtor Basic 500 Go bought on Ebay for 75 €
My previous one is a Laptop disk transformed on USB disk when my laptop has burned (Packard bell is not the best for BBQ)
To change the Letter, execute command (([Flag] + R) or Start->Run):
You are now in the 'central' command
-->give you the full list of command
list volume : give you the list of all volume monted on windows.
check your volume number of your USB disk
select volume 'Volume Number'
-->for select your volume number define in the list
Assign letter 'Drive Letter'
-->for assign your new drive letter.
WARNING: you must restart your computer, it's windows, in fact, no choice.
If you have another solution in direct 'live' for forcing a drive letter directly on any computer, you can add this in commentary.
PS: this is principally for me (no forget, no english man).
UPDATE !: After some tests, you can use also diskmgmt.msc which most easily to beginner. Just choose your drive and use right click -->Change drive letter. Thanks MS. No change, no choice, reboot for active drive.