I bought a 1MB Mapper/MegaRAM SD cartridge for my MSX 1, to expand the RAM & run games from SD Card. I was aware that the particular MSX model I own has compatibility issues with some mapper cartridges (a Sanyo MPC-100) and indeed the cartridge doesn't function - "Disk driver not found. System halted." error.
So I bought another MSX 1: (Stock web image, as it hasn't arrived yet.)
The more common (here in the UK at least) Toshiba HX-10, should work nicely with the cartridge.
I like the look of the Sanyo MPC-100, so I'll not sell it on. It can keep on happily running Cassette-based games & .CAS images from a CASDuino device.
Was so tempted to get an MSX 2, maybe in the next couple of years. 🤣