I am collecting some info on similar issues. Can you check something for me? When you format the card in camera, check the “SD card memory available” before and after formatting. Before formatting, do you have a low number (eg. 1.x GB) available out of 29.x GB? The camera may report “formatting complete” but afterwards, it still says 1.x GB available. If the formatting was really successful, it should show 29.x/29.x available (IOW, the card is empty).
If you are seeing the same low available space number after formatting, then I believe your card is damaged and will not be able to be formatted even on a computer. If you want, you can remove the card and try to format it using the tool here:
The SD Memory Card Formatter formats SD Memory Card, SDHCMemory Card and SDXC Memory Card (respectively SD/SDHC/SDXC Cards) complying with the SD File System Specification created by the SD Association (SDA).
I think you will find it report errors and fail to format. In this state, you can copy off what’s still on the card, but cannot format the card or write new data to it.
Let me know if this is what you’re seeing. This is what I’m trying to track down.