Can they be used simultaneously? I recently acquired a blank 4k cart and was curious as to what the hell I can do with it as I'm a complete MSX amateur. Hoping they both can work at the same time and maybe I can save games onto it? How can one put something onto the blank cart? and what could you possibly put on it if its only 4k? Any help would be greatly appreciated.

if your 4k blank cart are the sony HBI-55 you don't read/write directly in the s-ram because the cartridge use a chip 8255 for communicate with MSX

You have a second cartridge port on the back of the CX5M. But its only a connector, and no real cartridge slot. To use it you will need an adapter that Yamaha manufactured. Not sure how easy it is it to find one.

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You have a second cartridge port on the back of the CX5M. But its only a connector, and no real cartridge slot. To use it you will need an adapter that Yamaha manufactured. Not sure how easy it is it to find one.

And that thingy is on his way to the topic starter.

That 4k cart is pretty useless, it's very small and not standard.

The CX5M only has 32kB ram. I'm not sure how well the 64kB RAM cartridges work (I have one but haven't really tested it much). If it works as well as internal ram, I'd suggest you use that in the second slot.
In fact I'd like to hear if someone uses external RAM in their CX5M's. I have three CX5M's and it is my favorite computer but so far 32kB has been enough for what I do.

I'll let you know if I can ever get a RAM cart to plug in the back. If I was to find one, say a 32k cart, this would make the computer 64k - could I then install dos?

Yes. But it'll be easier / more convenient to find a bigger RAM expansion with mapper, such the one sold by Leonardo Padial (up to 4 MB = 4096 KB). Ideally, you should have a slot expander, RAM expansion and either an external floppy disk drive (noisy, unreliable) or an IDE-CF adaptor (Sunrise). In this ideal scenario you'll have a full featured MSX with plenty of RAM, hard-disk and DOS2 ready to go Plus several slots available to include further expansion devices, such as other sound generators (SCC, FM-PAC, Music Module, MoonSound).

Is anyone still selling something like that? I'm very nervous to send hundreds of dollars to another country for something I know almost nothing about (who wouldnt be!?)

IDE-CF can be bought reliably at Sunrise.