#4. "RE: D5100 and Lexar Platinum 11 SDHC memory card" In response to In response to 3
I bought a high capacity card ad a photography show and had the same problem. It was the house brand of a local camera chain so they exchanged it without question. For a replacement I got a two new cards (I think they were SanDisk). Befor using it to take any real pictures, I put the camera in continuous mode and proceded to run out a thousand or so shots on each card to fill them up to be sure they were completely good. (I had tried this with the bad card and it sometimes failed a few shots into the run, sometimes a couple hundred shots and once at about 800 shots). So, if you don't feel comfortable with the card, you might want to try that.