My wife's GS4 was doing the mounting/dismounting thing with my old 32 GB SanDisk Ultra (class 10) card that worked fine in my Note 2 for a few months before I upgraded to a 64GB card, so I gave her my 32GB card. I kept it in the phone, but just manually dismounted it so she wouldn't see it anymore (good thing I got her a 32GB GS4 so she had plenty of internal storage too), but it would still occasionally freak out when she let the phone die and then booted it back up. But I just installed the 4.3 OTA update today and it seems to be functioning fine now. I haven't seen it spontaneously dismount the card at all in the past few hours and I was able to use it to store pics on from the camera. So maybe this was an issue with the GS4 and not the SD card? Like I said; it worked perfectly in my Note 2 for a few months and I tested/formatted it on my PC several times to ensure it was a good card, but her phone still freaked out with it consistently before the last update. Its only been a few hours so far though, so it could very well start wigging out again, but I'll definitely post back if it does. Also, her GS4 worked fine with some old 16GB cards that we had from previous phones too, so it seems that both the phone and the card were at fault somehow.
Sent from hardware running software.