Can't load or save ANY preset anymore

[Lizard of Oz]

I discovered today that suddendly I can neither save nor load any preset on the MRCC.

…

It seems whatever I do the MRCC has no more access to the SD card.

Can your Mac write data to the card? Maybe you tested that, but I can't tell from you description, you say you formatted it, but there still appears to be data on it, you say the contents were not cleared from it when resetting the MRCC.

1. I assume you have a backup of our old card, if you haven't, and the data is still on the card, back it up.
   
2. Then, if you haven't already done that, check if the card is writeable: write a file to the card, then open that file from the card. If that doesn't work, you need to try a new card, go to #6
   
3. If the card is writeable, format the card using the mac disk utility (maybe that 3rd party utility is weird?), and verify it's formatted and empty.
   
4. Take a good hard look with a flashlight inside the sd card slot to see if there is any dust or such blocking the contacts
   
5. put the card in the MRCC and try again.
   
6. get a new sd-card (IRRC <=32GB), preferably a new pre-formatted one, you can get them at any supermarket really cheap, don't put the card in your mac, but right into the MRCC, and see how that goes 
   
7. if that works, copy over the files you backed up in step 1 (check the hidden files, your mac might write some crap to the card you might have backed up, you should delete that)
   

Skip right to step 6 if you're in an hurry.