Kevin reminded me of this-

https://retrogames.biz/support/the400-mini/upgrade/

Well worth it! It fixes M.U.L.E.!

See also, to add to USB in the back-

https://archive.org/download/Atari800RomCollectionByGhostware

And

https://www.atarimania.com/machines/atari-400-800-xl-xe/top

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