It's a problem that many people have had and most don't seem to be able to get it to work every time. I'm one of them. Love the computer.... in fact of the 10+ Garmins I've owned it's probably my favorite but the not mounting thing is ridiculous and I really think it's something Garmin should be able to fix with software.... although, I could be wrong.
Some USB cables are designed only to provide power and not data. Unfortunately, there isn't a standard indicator.