I HAVE had this problem, I didn't have it with the default card, but I switched after about a week or two of having the phone to a 32 gig. The issue is it randomly decides there's no SD card or that it's in an unreadable format, and the only solution is to mount the SD card to your computer then back to the phone (sometimes this works just plugging the phone into the computer, other times it requires unplugging the SD card and plugging it into an SD to USB adapter). This tends to happen every week or two, though I would like to note it hasn't happened yet since I installed mtmichealsons ROM to my phone.