Hi, I just spent hours trying to get my ATV silver to mount my Hard Drive. The hard drive is connected to my airport extreme via USB. Airport extreme is connected to Apple TV wirelessly. I followed all directions to no avail, until I found the solution (for me anyway).
The Drive would not mount until I changed the name of the drive. It was "Movies and Shows" I took out the spaces and called it "Media" and with everything else as suggested in the tutorials, etc, it worked.
So, something to try if you are frustrated like I was trying to mount an Airport Extreme USB drive.