It's simple, once you know the "trick". Right click the link on your desktop that corresponds to the USB drive, I'll assume /mnt/sda1 in this case. Choose Action and then Change read/write mode. You'll be asked if you really want to remount as writeable so choose yes if you do. That should be all you need to do to be able to copy your file to the thumb drive. Remember though, unmount the drive before you shut down to ensure you don't corrupt anything (has to do with write cacheing). Again, you can do this by right clicking the desktop.

'Goon