Good to hear you figured it out.
I've not found the F8 to be picky about SD cards. Still, Since it's quick and easy I always run the F8's SD card quick-test whenever I put a new card in, reformat, or delete files off a card without reformatting it. Not sure if that would have caught your issue or not though. Did you try the quick-test prior to reformatting?
In contrast to that, mostly because I've never had a problem with SD cards in the F8 and never wanted to wait for it to complete a full read/write-cycle for the entire card, I've never run the F8's SD card full-test. Presumably that may have caught the problem. Might be a good practice with new cards you aren't sure about.