I have almost the exact same situation. My card is a 32gb UHS-I card. I tried all the things DaveHo tried. I also tried formatting the card as FAT-32, and that also did not help. Thankfully, I was able to get my pics/videos off the card via a card reader on my PC before it seems to have become useless. For now, I am using a 16gb Sandisk card in my GS3.
I will be trying a warranty replacement. Hopefully that goes well.