@darthpaul
i guess you followed https://github.com/RetroPie/RetroPie-Setup/wiki/Running-ROMs-from-a-USB-drive ?
what's the output of df?
if you navigate to your USB driver, what are the contents if you do ls -l ?
what is the output of ls -l /dev/disk/by-uuid/ ?
can you paste the contents of /etc/fstab?