I find that my USB key is pretty consistent in what drive letter it gets mapped to. My regular key always seems to show up as G:\ at home. but as I:\ at work because there is a network drive mapped to G:\. A different brand of key I just plugged in at home showed up as F:\.
I don't recall the last time a given key showed up as a different letter than usual, but I'm sure it has happened before.
I think your best bet may be to use a file path control that would let you browse to and select whatever drives show up on the system.