I was shooting with my Nikon D7000 a week ago and got a "CARD Err" message. Even though I'd been shooting with that particular SD card in the first slot for a while (and had used it previously and reformatted it multiple times), I took the message at face value, turned the selector switch off, and pulled the card, replacing it with a card that is on the Nikon list of approved card (maker, size, etc.), that had been formatted in the camera and I'd shot with previously. Still the same message. Hmm...I thought. Tried another card. Same message. Another card. Same message. Another card....same message. I went through 7 SD cards (all of which had been formatted previously in the camera and had been used multiple times in the camera). 5 of the 7 cards were on the Nikon list of approved d7000 cards. I consulted the d7000 manual. I checked the firmware (yep, I've got the most recent version). I made sure none of the SD cards were write-protected. And no, none of the cards were full. Now, a couple of days later, my D7000 acknowledges all of the previously rejected d7000 cards just fine and I did test shots with each card--again, A-okay. Does anyone have any advice/theories about what was going on here?