FYI:
Change and Assign Drive Letter in Windows 10

- and to note, as you probably know, if you assign the same drive letter to a further device, the first one loses that assignment (which I think is what Try3 meant by 'once only').

From time to time there have been proposals to change the disk id so several external disks have the same id, and can thus be used interchangeably, being assigned the same drive letter.