Wiring umass unit numbers

[george+freebsd at m5p.com]

I have a USB floppy drive and a USB flash card reader/writer.  At
various times, neither, one, or both devices are plugged into the
computer.  Depending upon when each is plugged in, the floppy
drive might become /dev/da0 and the flash reader/writer /dev/da1,
or vice versa.  Is there a way to specify which should be which?
-- George Mitchell