How are you creating the course from an historical ride on the unit or from another application/web site.
What I used to do, was to create a course, export that as a tcx course, copy that to the new files folder, when you turned the unit on (500 in this case) the unit converted it to a course file which was a fit file. I currently use a 705 which use different file formats.
I also use other garmin offline software, Garmin Training Centre, its a bit old now but does what I want, I tend not to trust things that automatically sync things, as you are not 100% sure what is going on.