I have a 520 and this message suddenly appeared when I was trying to load any of my courses that had been on there for ages. I couldn't load any course & it wouldn't delete them either. So I called Garmin Help and we found the problem - the 'courses' folder had disappeared and been replaced by a corrupted 'courses' file. Here's how we fixed it:
1. I performed a hard reset (details online, but basically turn the device off, then hold both 'start' and 'lap' buttons whilst restarting the 520.
2. Plug into your computer (I have a Mac)
3. In Finder, click on your Garmin and see if it has a 'Courses' folder - mine has been replaced by an "exec" Courses file, which was presumably corrupted. Delete this file (drag to trash, then empty trash).
4. Eject the 520 & restart it
5. A new Courses folder should be created and the problem should be fixed.
Along the way, we also deleted the 'GarminDevice.xml' file, though I don't know if this made any difference - a new one will reappear when you restart the device, if you do this.
Hope this helps!
