I am not sure what has changed, but on Server 2012 I have seen this annoying issue happen now multiple times and can not resolve.
If you have say 3 USB drives and you plug 1 in at a time and assign then all to say the M: drive. The logical assignment will stick for the drive currently connected. Then when re-connecting the original drive (should be M:) the next logical drive letter is assigned. This is the the original drive in the set.
So basically the Disk Management does not let you assign the same logical drive to different drives even though they are not connected. This was not the case before. I have been able to for years script a backup to say the M:\backup folder and make sure it was on all 3 of the USB drives. Then you can rotate the drives at will. Seems Microsoft has taken my logical drive letters and the start menu at the same time :(