After today's ride, I plugged my edge 800 into the PC to upload it.
Nothing.
Activities folder empty. ( I cleared it out last night before the ride. )
On the device, activities list empty.
It *had* been recording, so I tried using an undelete utility to see if the file was there, but marked as deleted.
Bingo!
There it was.
Un-deleted it and it was fine.
Stupid machine.
Edit: The device was on the brink of death. Next time I turned it on, it stuck at Locating Sally's Tights. Even though the GPS menu showed good signal strenght from many tights. Master reset didnt help. Re-flashing the firmware has brought it back from its near-death experience.
Since it won't re-flash the same version it has on it, this required me to load an older version then reflash the latest.
But its now working again, for now.