Retropie 4.7.1 Official. Simple tutorial using a USB drive for roms and emulators. Part 1. Raspberry Pi 4 2GB.
Works with Pi 4, Pi 400 and more also supports USB boot
I was using a 2gb Pi 4.
Added PSP and Dreamcast emulators
My Folders for running roms from a USB device. Google drive
Unzip and add to the root of you usb device. Format as fat32