I have one of these awesome devices:

but just got nailed by a nasty little issue that resulted in me losing 7 hours of data. Just wanted to share a warning/tip with any other G500 users out there. 

I failed to charge the unit overnight after doing a 7+ hour ride, and fired it up again on day 2. About 7 hours in it said "warning, battery low" and about 5 minutes later it shut off. No big deal I thought, it saved its remaining battery and I was going to not record the last 30 minutes of my ride. 

Turns out that it DID NOT record ANY of the 7 hours - the shutdown failed to save the data. CRAP! 

I asked Garmin about this and got confirmation: 

"Thank you for contacting Garmin International. I will be happy to assist you with this.

If the device powered down unexpectedly, the Edge 500 most likely would not have had time to properly write the file. This could have caused the Edge 500 to lose the data, resulting in the lost data file. Have you connected your device to the computer and verified the file isn't showing up? I can help you check this if need be.

Unfortunately, if the file isn't there, we won't have any way of recovering it.    

Please let me know if you have any additional questions. Thank you and have a great day!"

So - note to all users - once you see the "battery low" message, shut down the unit yourself to save the file its currently writing. Maybe hit stop on the timer 1st and then shut it down, as you'd normally do after a ride.