I have had this same 'Cannot mount volume' message when trying to swap files
from my Windows PC to the Ubuntu box with a USB stick..

Turns out the problem occurred when I simply 'unplugged' the USB stick from
the Windows PC..

I doscovered if I first dismounted the USB stick using Windows 'Safely
remove hardware' option t, Ubuntu accepted it without problem.

Maybe if you dismount the USB reader before removing it from your computer,
it will continue operate as expected?

I am working with Ubuntu as the only operating system on my Toshiba
laptop.

I have been using one of my Nikon D1X Lexar CompactFlash cards in a usb
card reader for temporary backup. So long as I kept it plugged in it was
working fine.

When I removed it from my usb reader and tried to put it back, I got an
unpleasant surprise. Cannot mount volume. The Volume 'Nikon D1X' uses
the FAT32 file system which is not supported by your system.

In the camera I could still see my Nikon photos, but all my backup files
were invisible.

After a day of scanning through the Ubuntu Forums and trying all kinds
of "solutions, I did find one temporary fix.

Restarting from the Live CD allowed me to open the usb compact flash and
see my images.

I don't know what that means, and I am still looking for a permanent
solution.