I've heard of this problem and I think the problem is because the folders your camera puts on the card are missing so that's why it wont recognise it.
So if for instance you normally have a 101DCIM folder and a MISC folder you need to manually replace them via your computer and then hopefully the camera will recognise them.
I don't know if this will actually work as the person who told me about it had deleted the folders from his card rather than formatting it but it may be worth a go.
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