Add 'user' to the 4th column for the line in /etc/fstab that is your usb drive.
I'm not sure if that makes much sense, so here's my entry for that device.
(Mine is /dev/sdc)
/dev/sdc /mnt/thumb msdos user,noauto 0 0
Replace /dev/sdc with your device name. Then, remount it. Make sure you edit the file as root.
Hope this helps.