All I’ve needed to do was point es_systems.cfg to the new rom location. Another possible solution is to create a symlink in place of or inside the default rom folder that points to the media/usb0. I’ve used both methods.
One thing I did notice is that you need to format the drive or usb stick in such a way that it’s fast enough. (and I assume you want to be able to read and write to the drive via windows) I used ext4 on a 1 TB usb drive I recently starting using with my pi.
^^ my other hobby