I know this is several years later, but I just ran into this problem and in case others search here, here's what I found. I had deleted all the photos from the memory card using move to trash on my Mac. But while the card was loaded on my camera or my Mac, it saw it as being full. It wasn't until I used the Mac app Disk Inventory X that I noticed it was saying the files taking up space on my card actually resided in the trash. Once I emptied the Mac trash, the space on my card was freed up.
