Thanks everyone for the suggestions. I did format the SD card using Windows this morning and then copied the files again, but still got the same error message. Looks like I might have to return it to Devialet. They still haven't responded to my ticket yet.
Deviate support got back to me yesterday. Apparently the ticket had gone to a wrong queue. They think that the DUET board could be sitting loose or faulty. They asked me to unplug and re-plug the unit .I had already done that multiple times. Waiting for further instructions. Hopefully we can resolve it soon.