According to Sandisk support when I talked to them around June, 2013, and all over the Amazon reviews for months, is a known incompatibility between the Sandisk Ultra Micro SD cards (possibly others), and the Galaxy phones (possibly others). The card can "fail" any time from days to many months down the road. Sandisk says that Samsung won't admit it, and so Sandisk created a ROM patch that is applied only during manufacturing, to circumvent the problem.
Contact Sandisk for free replacement. I was told that the new card batch started after March, 2013, but the only way to tell, is based on a small serial number on the card, that requires a magnifying glass, if you can even then read it.
I do not know if other cards from other mfgs are involved, but Sandisk says they have stepped in and solved it on theirs. Good luck!