I reformatted the drive to ext3 and got exactly the same problems as
before (owner:group reported as root:root rather than the root:plugdev I
had set them to). I sudo chgrp'd /media/Portable (where I have the
drive mounted) and was told I didn't have permission to change the
group, so I sudo'd into Nautilus, navigated to /media/Portable,
right-click on a bare part of the file list window and was allowed to
change the group. Why I should be allowed to do it this way and not
from the command line, I have no idea - but it worked, so I'm not
complaining