If you have a raspberry pi image you can just pull all the rom folders off and put them on a flash drive. RetroArch will load them all easily since it's based off of Libretro just like RetroPie. Although the interface is less user friendly and takes a bit more poking around, it works great once setup. I have like 1,000 roms on mine for all the 8-bit/16-bit/arcade games.
If you have a raspberry pi image you can just pull all the rom folders off and put them on a flash drive. RetroArch will load them all easily since it's based off of Libretro just like RetroPie. Although the interface is less user friendly and takes a bit more poking around, it works great once setup. I have like 1,000 roms on mine for all the 8-bit/16-bit/arcade games.