I recently had a problem when I manually deleted files on my card while connected to my MAC. My normal empty card (16 GB) could shoot 58 minutes in 1080, but now was only showing 13 min available. When back in the camera there were “no files to delete”.
To fix it I had to shoot a quick shot to that card, then thru the camera menu / systems, I formatted the card. After that it went back to having the full 58 minutes of space available.