I have no solution to bring, but just wanted to add a “me too” to show the problem is still occuring
My card is a microsdxc sandisk ultra 64GB that I have been using with my gopro hero3 without any issue for nearly a year (purchased April 2013). It only just started to give me problem: the gopro now gives me a “SD ERR” error, but using it on my phone or PC, I have the same symptoms you describe: I can read whatever is on it now, and it *looks* like I can delete files or format, however as soon as I plug the card out and in again, it’s back to the old state. I’ve tried formating from my phone, and from by ubuntu PC (fdisk+mkfs.exfat), nothing works
I have sadly misplaced the purchase receipt, but I still have the packaging that states the lifetime warranty. I hope that’ll be enough to get me a (working!) replacement.