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?
Jan Van Mieghem 
Ah those techies—always looking for ways to confuse we lay-people!! Da
Mac always leave hidden folders here and there, especially USB drives, garmins included! Don’t be surprised to find 5yr old files in ./trash
Didn’t know so Good to know, Jason. Thanks. I will be mindful of also checking USB etc
It’s not your laptop’s trash, each drive (your Garmin appears as a drive to the Mac) has its own trash folder. But useful to know anyway.
I came across this when trying to find out how much data the 510 will hold before it begins deleting old files.
Bob: I’m no Mac expert, but on Yosemite I only see one trash can for the entire computer. When I empty my trash, it also seems to permanently delete the “deleted” fit files on the Garmin 510 because by right clicking “get info” on the Garmin (mounted as a drive), I can see that I have more memory available again. (Probably “delete” on Mac is just a temporary delete to allow you to “undelete”.)
Garmin’s software capabilities are sometimes a mystery: my Garmin 510 now shows ” Capacity = 21.8MB and Available = 30.5MB”. It also says “Created Wed, Dec 31, 1969.” Go figure…
Wow! This is so helpful Jan! Thanks