During two big rides this season my Garmin 510 ran out of memory and I lost the data. Bummer! The weird thing is that I always delete my activities so there were no old rides on the Garmin so how could it have ran out of memory? Where did the 10 or 11MB go?
I may have found the weird answer from this Garmin forum:
If you are using a Mac then you need to empty the trash while the 510 is connected to the Mac to really free up the space.
So I did and, indeed, the Garmin 510 then showed again 10+ MB free. Hopefully this will solve the problem, but who would have figured that you need to empty the laptop trash to clear Garmin memory?