Everything seems okay now. I had a Linux system I needed to set up. (Others were in use and I couldn't easily use them for this.) I got it up, added GPartEd (which, interestingly, did not include GPart as a dependency) and read the card from there. GPartEd could not find anything wrong with the partitions on the card, but after I used it to read and very, the card worked when I put it in the Pi!
I didn't write anything to this device, in Mac or Linux, so my guess is that GPart or GPartEd did something while just reading. Maybe it had trouble with a few bytes or when it wrote a date, it updated something else. I don't know, but just reading it and having GPart look at it was enough that the card started working when I tested it.
It's worth noting that when I booted it with my Pi, the system did have issues with partitions and scanned them, but found no errors.
I'll be taking it down and testing it in the arcade setup this evening.