You could set armory's data directory to your flash drive. It would have more than just the wallet file though. That would include all of the logs and databases that armory generates in addition to your wallet file.
Thanks for your answer.
I'm not quite sure if it's a good idea to put the whole subdirectory (about 50 GB) with the armory database on a USB stick. The startup of armory is already rather long and I don't want to slow it down to hours.
BTW: I found the "wallet.dat" is in the bitcoin directory which is about 60 GB large. Or is the wallet not in "wallet.dat" but somewhere else?
eParanoid
That would be Bitcoin core's wallet file. You have in essence two wallet programs, one is Bitcoin Core and the other is Armory. The one with all of your Bitcoin is the Armory wallet, and that is the one you should care about. The wallet.dat file is for Bitcoin Core and that one is irrelevant to you. The wallet you want is in the Armory data directory.