there is a blurb on the LSDJ wiki about the RAM size limitations and what it means, this seems to be somewhat confusing to folks. copy/pasta typos included:
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'Q:' What are the compatibility considerations to be aware of, regarding different models of transfer devices and different brands and types of cartridges?
'A:' 'Cartridges:' The cartridge ROM size should be 8 Mbit (Possibly 4 Mbit if you're ok with using an old version) This is usually no problem since most carts have more than that. As for RAM you need 32 kbytes to run LSDj, and to be able to use the file system, you need 128 kbytes. Many carts do not have 128 kbytes of RAM, so watch out! Also, some carts do have 128 kbytes of RAM, but do not support acess to all of it at the same time. In this case yo can have four copies of LSDj each with one song, but with no access to file saving.
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in short: filesystem will be disabled without 128K, each 32K sav is enough for one song. so 16 songs on the 256M cart, for example, assuming you used all the slots for LSDJ savs.