I just experienced the same issue with two SanDisk Ultra cards. The camera said it could not access the card and I should replace the card or format it, but the camera would not format either one. I took the cards to my computer (a Mac), inserted them into an SD card reader, and both cards opened without any issue. I did not do anything else after opening the card. I ejected the cards from the computer and tried them in the camera again, and they both worked. I have no idea why. Something about opening it on the computer fixed the problem.
