Double check the connections on each end to make sure they are nice and snug. If possible, plug the Nano into another existing USB 2.0 port.
Also, with the iPod still connected, try resetting it. To do this, press and hold both the Select (Center) and Menu buttons (or the Volume Down and Sleep/Wake buttons on the 6G Nano) long enough for the Apple logo to appear.
Then try restoring the iPod again.
B-rock