I thought I lost a file on my fenix to battery death, but it ended up in a different folder (and not as a fit) - if you have yours configured to appear as USB maybe that's an option.
I forget exactly where, maybe instead of Activity under \GPX\current\ or something.
yes, what Mr. W said, i've looked in to the Garmin as a USB storage device and found previously un-syncable files.
also sometimes Strava will pull it out.