Uh, really? Why is there a "Dump USB device" function then?
I saw there are two versions: wfs-extractor and wfs-file-inject so that's kinda weird if you can't dump data from USB...
Look at this thread though!
xD You can dump but you can't decrypt, WFSlib cannot permit it for the moment. You can inject using wfs-file-inject and it's not wfs-extractor, but wfs-dumper.