Mine has also worked briefly and then stopped, as indicated by an X on the camera icon instead of a red dot. At first I was using an older 8GB flash drive. It worked for awhile on 39.6, and then I noticed once I had updated to 39.7 it was broken. I have read that the update may have corrupted it. So I removed it, reformatted and tried again. It worked again, but once again some time later it stopped.
Today I bought a new 32GB flash drive, and the story has been the same. After setting it up it worked, but I noticed tonight after an evening out that when I returned to my car to go home, it was not functioning yet again.