@comet424 You can use the SSD for OS also, but it will be formatted with a Linux filesystem and you'll not be able to read it from Windows/macOS. Using a sdcard for the OS/RetroPie is easier for upgrades, since you'll write the new OS release (when upgrade will require it a full re-image), but you'd still have the ROMs on your external SSD. Using the SSD with a Windows friendly filesystem will also make it easier to copy your games to/from it on another PC running Windows/macOS.