I was able to find a PC to put the software on it again. When I did that it got corrupted so bad that no device would read it at all! I couldn't even format it on my PC
When I got back home, I returned the faulty card (Sandisk 2 GB MicroSD) to the shop. They thought it had to do with magnets too. But I got a new one.
Three weeks later, the same thing happened!
When I tried to run SD Formatter 2.0 on it it worked. Since then (about 6 months ago now) the card has worked flawlessly in my M3 Simply and M3 Real.
My advice: format with SD Formatter 2.0