Brandon --
the card had some folders at the root level alongside the DCIM folder. I deleted those, emptied trash, then reingested. this time the ingest worked perfectly.
I'm sticking with my theory that the card was out of write space that PM needs to keep track of what was ingested.
It would be great if a future release of PM would actually specify what the error was, rather than just saying that there was an error.
Yes, I do have another card reader, but I needed to clear this card. Life goes on, and I couldn't preserve the test variables.