Changed fstab to:
proc /proc proc defaults 0 0
/dev/mmcblk0p1 /boot vfat defaults 0 2
/dev/mmcblk0p2 / ext4 defaults,noatime 0 1
UUID=1918-2A41 /media/usb0 vfat auto,nofail,noatime,users,rw,uid=pi,gid=pi 0 0
/media/usb0/BIOS /home/pi/RetroPie/BIOS none auto,nofail,noatime,users,rw,uid=pi,gid=pi,bind 0 0
/media/usb0/roms /home/pi/RetroPie/roms none auto,nofail,noatime,users,rw,uid=pi,gid=pi,bind 0 0
more streamlined and I think 'bind' in the first of the three additional lines was the cause.