I find different cables can make a difference, if you haven't already tried that.
Also if you believe it might be due to getting wet, let it dry out for a while (maybe in rice). I got my 500 soaked a few times and it had 'difficulties' right after the ride(s) but perked up a few days later.
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I don't remember exactly how to do it but I'd try booting the Garmin into disk mode, copying the Activites folder, and uploading to Strava.
Also, I was able to mount my corrupted iPod to a Linux machine when my Mac wouldn't mount it. It was read-only but I was able to copy most of the files off of it, despite the HD having some major issues. Perhaps it was just luck but I'd imagine it's less picky about that stuff than OS X. If you have access to a Linux machine, it's worth a try.