It has come up a number of time with failing USB drive backups that i have needed to change the letter of a drive to make it all work. On one server every day it was wrong, so we needed to log on and fix it every day. Now we have a script.
Save as Diskpart.txt
select volume 5
ASSIGN LETTER=h
Edit to reflect the volume and the drive letter you need to change. Save it to a location that the script can call.
Save as Diskpart.bat
DISKPART /s C:diskpart.txt
Make sure the path reflects where you have saved the diskpart.txt
This should allow you to change the drive letter automatically every time its run. Either put it in a for script in your backup program, or run it as a scheduled task everyday.