My AT&T US version I couldn't get the phone to mount via USB only charge...I tried everything, what I found out to be the problem...the cable that ships with the phone. When I used the supplied cable it would only charge the phone and not mount the drives. When I used the USB cable from my other droid phone it worked fine. Oddly enough I tried the Samsung supplied cable with my other droid phone and it didn't mount either, just charged.
Try a different cable.