I've searched and read, so I thought I'd throw this out there.
I went to go for a ride today, and my 520 wouldn't start. Did some searching and found that just holding down the power button would give it a reset and start, which it did. I set everything back up, paired power & heart rate, and went for a ride. It worked perfectly through the ride. When I got back, I plugged it into the USB cable in Mac running the latest 10.11.1 and it would not recognize the 520 as mass storage, nor would Garmin Express recognize it. I tried 3 different cables, and two USB ports with no effect. I was able to get the ride off of the Garmin via bluetooth with my phone but it seems there's no way to get it to communicate with the Mac. (Old Edge 500 synced fine with the same computer today)
On a side note, it's been charging for sometime, appears to be hung at 42%, and it is quite warm. I'm guessing it's close to death, but I thought I'd see if anyone had any ideas on getting it going again. It's been reset a number of times.
