My S4 has had the same 32gb chip in it from the weekend they were released. It's a SanDisk Ultra. Never had any issue with it. The wife's S3 has a Samsung 32gb SD card, and it's also been trouble free.
I've read that the quality of the SD cards has been down of late, so it could just be some bad chips.
I had a issue with a SanDisk Ultra 32 GB MicroSDHC C10 card, it would keep unmounting and mounting at all times. Switched in a Samsung card from my tablet and the issue went away.
Which brand of card are you running?
On a side note, I sent in my SanDisk card for warranty and they are replacing it for me.
One is a Lexar and one is a polaroid both just puchased withing the last few months , after saying it was damaged it still allowed me to reformat and then work fine after, just lost all info thats all. Needless to say I will be saving camera shots to phone storage instead of card just music is going on there not taking that chance again !
OP sorry to hear that. I had a problem just over the past few days, where my SD card would unmount itself and then became damaged. I was using my phone to record and take pictures for Christmas. (I have the SanDisk Ultra 32 GB MicroSD HC I card which I bought in Oct. 2012.) I was able to recover all my files/data off the card using my computer. I called SanDisk and they will be shipping me a replacement. Just a suggestion, regularly back up you phone.