Originally Posted by GDroid
I have a hunch the OP is not "mounting" the phone as craighwk described in his reply. The same cable is used for the wall charger, and the OP should have noticed issues with charging as well. This is assuming the OP is not using a different cable when connecting Droid to PC.
I have that hunch too. That caught me by surprise the first time I tried to hook up via USB, but I did look at the phone and saw it was connected. I did a quick search and bam the answer was provided. gotta love the intertubes-webby thing.