I'm not sure if it's the phone, but I've gone through three 64GB SanDisk Ultra microSD cards.
The first didn't work at all, and I exchanged it. The second worked in its "native" format (exFAT) in my stock GS3, but I then installed Cyanogenmod 10 (which doesn't support exFAT). I tried to reformat the card as FAT32, but it became unreadable. The third successfully formatted to FAT32 and allowed me to transfer *some* files (in Windows), but then started to give error messages, both in Windows and in the phone. It then became unreadable and unformattable (in the phone, in Windows, and in Linux).
I've given up and ordered a 32GB card, which I hope works better.