SD cards some times seem to shrink or just get unreadable. Most of the time there is nothing wrong with them at all. They have just been written to in a bad way that messes up the partition table or breaks it all together. Most of the time you can fix it with disk management tools but it's more work then needed. I usually get problems with my SD cards from bad images for my Raspberry Pi but even my GoPro camera messed up a card ones. I also use a Denver action cam that usually formats the SD card with a smaller partition then the actual card size. SD Formatter from SD Association fixes the cards every time. Just run it against the card and turn on "format size adjustment" and it will come out just fine.