I already had that problem when the FAT filesystem on usb flash disk was corrupted.
Unplug your flash disk, fire a "tail -f /var/log/messages" as root in a terminal window; plug your flash disk.
Check the messages on the terminal to see if there is any problem when mounting the flash disk.
If there is, copy all data from the ready-only device to a safe location somewhere in your home.
After you are sure you have a good copy of flash disk contents in a safe location, just format it.
This use to happen when you unplug your flash disk without unmount it, or in the MS Windows, without using that option on system notification area, to eject safely your disk.