I have been tearing my hair out trying to resolve the Sim stopped, Sim unmounted error codes. I have even exchanged the phone once only to find the new handset to have the very same issues. First of all it seems only to be isolated to Verizon S3s. I have friends with other carriers running different brands of class 10 cards with no issues at all. Here is what I have found ONLY SanDisk cards work properly in a Verizon handset. I have no idea why but every other brand I tried has some sort of failure message with the Verizon S3 handsets. I purchased one card that the phone would not even write to at all even when formatted in the device but would work fine as pie in my digital camera. What is different with SanDisk I have no idea but one thing I can tell you they work 100% in this handset no matter of the size or class speed. I am now running a SanDisk ultra 32gb card with not a one error message in days now. Costco has the best deal on them it was only $23.