What Cap'n B said.
I have four plug-in USB hard drives, backup for the use of. Each appears with its allocated drive letter, regardless of which of the others is plugged in. There's a file in the root directory of each as well, saying "THIS IS DRIVE R" (or whatever) but it's never been needed.
USB sticks don't seem able to do the same thing. They all appear as Drive O, unless there's already one there. My Panasonic Camera is Drive P (keeps it simple).