I only have a zaxxon DIVMMC and that saves fine when pressing R (I think, but possibly S - not used it in a while). I'm pretty sure I had to set jumpers on it for the correct kind of spectrum before it worked though as it supports 48 and 128K. It's worth checking they are ok on yours if you have any.
Also load and saving will undoubtedly use different tracks on the edge connector. Give them a good clean!
Presumably the sd card is ok? They can be problematic - on occasion I have had to copy everything off it, reformat it and copy everything back again to get it working.
Have you tried typing in a simple program and saving it to the MMC? If that does work then I would imagine it is not the card or the edge connector.
As for the picture.... I have the same - the composite via a scart adaptor is poor. The 'dot crawl' is terrible.
You could try getting a dedicated comp to scart cable (I.e. not a cable plus adaptor) which may help a bit but probably not much.
You could also try
http://www.fruitcake.plus.com/Sinclair/ ... bility.htm
Not got one or sure whether it would work with your MMC but you could ask the guy who sells them.
I solved my problem by buying a toastrack with the RGB out and it has a perfect picture....
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