There seems to be 3 different methods to accomplish using roms on an external drive.The official retropie recommended method is to copy the retropie folder to external drive and link it using fstabhttps://retropie.org.uk/docs/Running-ROMs-from-a-USB-drive/Anther method I've seen recommend is to to edit the es_systems.cfg file. And finally others have said symbolic links are the best.