Try backing up the contents of the SD card to your PC, reformatting the card as FAT32 with the PC, and then restoring the contents. Even though there are no errors detected, it sounds as though there's something "fishy" about the card's format.
I've found that some cards come from the manufacturer with a rather dubious format; it seems to improve the reliability if you format the card yourself.
Bit of a strange one, though!
From the reports we've had here, the occasional SD card just plain doesn't work with the Gen3, for no obvious reason.