2 
1 faulty Card slots not seeing cards or card is locked
My camera tells me my camera cards are locked or gives a card error
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263290
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Try blowing compressed air in different directions thru the card slot... after almost the entire can it came back to life on my D7000 like nothing ever happened...
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I'm having the same problem. I was told by Nikon Service Center - Need to replace the card slot, cost of it is too much.
