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If you read the mpselectmini.com website, there are a number of tips and tricks there.

Basically the card has to be an MMC spec card formatted in FAT for it to have a chance of reading the files on it.

I have the original card and I found the 2GB card recommended on the site both worked, but sneakernetting files was a pain for me.

This is why I moved to OctoPrint via a Raspberry Pi. No printing problems since then (that weren't caused by my own stupidity or design flaws in the printer itself) and I can remotely upload/slice files on the print server before printing and monitoring it remotely.