or you can just buy a 64gb memory stick and copy the roms/bios files to it and put in in a folder called retropie-mount. Samsung has one that is almost flush mount for $20 US and you have the bonus of plugging it into another retro pie and have it as like your own "cartridge" with all your own games and saves. Also if you ever have to reformat and start over you don't have to reinstall all the roms again.