Interesting. I just now ran another test plugging-in the same USB
thumb drive before I logged in; I wanted to see who the system
would assign as "owner" of the thumb drive.

Now Properties cannot show anything about the thumb drive, though
I can, after logging in, copy and delete files on the thumb drive.

Clearly, the system is confused, and this is with the latest patches
for 8.04 as of yesterday (Saturday).

Unmounting the thumb drive works OK either from the command line or
the graphical interface.