"This memory card cannot be used. Card may be damaged. Insert another card". I have watched the tutorials, changed to a new SD card, tried SD cards that are readable by a D750 and video camera, and am hopelessly stuck. I even researched updating firmware, however, I am only reading that you may do this through a SD card and not a direct connection to the computer. HELP!
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If possible, I'd try some really low-line card, such as an old style SD of a couple of gigabytes, or a slow SDHC card, and see if it works. If that does not work, I'd suspect the camera.
SD card contacts are pretty reliable, but if possible I'd look down in there and make sure there's nothing impeding contact. Maybe wiggle the card around a little.
You are correct that the firmware can only be updated with a memory card. It's unlikely that this is the problem, but for reference, make very sure you follow the instructions correctly for this. It's quite reliable if you do it right. You download the program and then unzip it, and put the unzipped "bin" file on the root directory of a freshly formatted card (not in the image folder). When you go to the menu item for firmware it will detect it, and ask if you want to update. Make very sure you have a charged battery. It does not take long, but it does have to finish reliably.