I put the SD card into my Ubuntu laptop, copied some files over, and then unplugged the SD card without ejecting it. I thought it would be safe because I hadn’t altered anything.
But when I replugged the card, it was invalid. And fsck.exfat on Linux knows nothing. It’s worse than John Snow.
I plugged it into an OS X machine, though, and said “fsck_exfat /dev/disk3s1” or something, and it said that it fixed it. Except all the MP4 files were truncated at 128KB.
So there’s no second set.