Using External SSD for Roms NesPi RPI4 case

[Emulatre71]

I have one of those NESpi cases that has a SSD connected to it, how would I be able to get Retropie to scan and use the roms located in that external SSD. Does it read or work with EXT4 File System Format? I do not want to format it because I can use it with my Lakka OS (emualtion Frontend OS) installed on another microSD card.
In other words I want to be able to swap between both OS's that are on separate micro SD cards that are using the roms located on the SSD. To be clear no I don't want to boot from the SSD.

[mitu]

You can use your existing ROMs on the SSD, you just need to mount it first - easiest is from autostart.sh (which you can find at /opt/retropie/configs/all).
Since RetroPie expects each system to be under it's own directory in the roms folder, you should probably bind-mount your ROM folders under the existing system folder. Something like:

# mount the SSD first
sudo mount /dev/sda1 /media/usb0

# make sure each system is in the place with 
mount --bind /media/usb0/NES_Games /home/pi/RetroPie/roms/nes
mount --bind /media/usb0/SNES_Games /home/pi/RetroPie/roms/snes
# etc.

You can go without the bind mount steps if you modify the es_systems.cfg, but I think it's easier to map existing folders via mount --bind if the folder names don't match.

The biggest issue IMHO is the filesystem's permissions. Since it's ext4, you'll inherit the existing permissions and you'll probably have issues if the pi user cannot read or write to the ROM folders. By default, in RetroPie save states and game saves (.srm) are saved in the ROM folder. That may pose a problem if the pi user cannot write there, though you can probably change the save location in the global retroarch.cfg file.