I'll preface this response by saying, obviously I don't know whether your card is a dud or not. I'm sure it happens.
But if you review the comments on this thread, you'll find that there are people that experienced the same thing you just did. First their card worked, then it didn't. Tom's response back then was that the card is probably old and it wore out, but people asked well if that's true then why does the card work in my DR680? (Response as I recall was cricket sounds.) Now you're concluding that you have a dud. Hmm, yeah it's a new card so that's certainly a possibility, but it's a strange coincidence that you're seeing the same thing that others have.
Anyway, I bought the same card as you did assuming that it would work fine and haven't used the DR70D since, so I'll do some testing to see what happens on my end. (Obviously, based on your feedback, now I can't assume this approved card will work, so once again my confidence goes down a notch so I gotta waste time proving to myself whether the card or the recorder is at issue. I was hoping I'd gotten past having to do that by spending (wasting?) more money on an approved card.
)
You won't be surprised by my response to this, but I don't think it's fair to assume it's not an issue with the DR70D and make Sandisc/Best Buy pay for the error that, in all likelihood (based on your experience repeating what others have seen), is on Tascam. Have you tried the 'dud' card in another device? Bet it works fine.
I acknowledge (and hope upon hope) that the card really is a dud, but I also don't believe in repeating coincidences. That said, I can't even begin to think of a technical reason to explain why your 'unapproved' card works and your approved card doesn't work now after it did at first. Maybe firmware v1.12 addressed this whole mess, but I doubt it since Tascam hasn't yet acknowledged that there is anything but an approved/unapproved card issue at work.