So this may be old news, but Doug emailed me back and says that 661s without the firmware update can't handle UHS cards, needs the V2.04 update. Mystery solved. Rather than take a chance on bricking it I'm gonna have Doug do the update, 6 year old deck could use a checkup anyhow. I asked if this was an issue out of the box with the MK II, which is my other deck, I'll report back on that. I've been using a 16 gb Lexar UHS card for maybe 50 hours on dat transfers with the MK II without issue, so I think it has the update.