I’d like just to point out, I’m not saying this is the way this should be done, this is the way I have done it & it works well for me
ok, here goes,
firstly I edited /boot/config.txt on a fresh RetroPie image.
sudo nano /boot/config.txt
adding right at the bottom
max_usb_current=1
next I format the drive to ext4 via a spare rPi running Raspbian, using gparted
sudo apt-get install gparted
next boot the RetroPie image I prepared earlier, with the drive in.
once set up RetroPie, I set permissions on the drive with, this will allow to add files as & when needed via ftp
sudo chown -R pi:pi /media/usb0
now I don’t use any uuid’s as I don’t plan on using any other drives, (I use a 128gb USB drive)
then i can create folders & add roms…
then edit the es_systems.cfg
pointing directories to
/media/usb0/