You might not actually be connected to ground. You have DC going from the power supply to the notebook. Depending on how the power supply is isolated or not isolated, grounded or not grounded, there may not be a direct connection to an earth ground.
Just to be sure, I ran it past a couple of my ESD experts here.
Connect your ohm meter between the ground wire you have coming out of that USB cable, and the ground pin on the AC power plug (with the AC power cord unplugged). If it's less than 1 ohm, you're good to go.
@Duane: In most US wall sockets, the screw that holds the outlet plate on is grounded. If that's the case, you could just connect to that screw for your earth ground.
But they I still have the probem that the wall socket is quite a way from the table -- unless I attach a loong piece of wire -- and I don;t fancy attaching and detaching that wire each time I want to play with my electronics...
Hi Max : I get over stretching the colied cable of the wristband by having a multiple socket strip, on a lead from the wall socket, in England these are very cheap from Ikea. The soldering Iron and powers supply go into two of the free sockests leaving me with two other sockets for wristband and antistatic mat.
In general, grounding yourself through a piece of (consumer grade) electronic equipment for ESD control seems like a not so good idea. You are making assumptions about the goodness of that ground and assuming the equipment can withstand discharges into that point. If you are sure only the wriststrap connection touches the "ground" point, then currents are limited by the resistor in the strap anyway and less of a worry. If you grab the "ground" to make your connection, you could potentially make a direct discharge at that point.
Having said that, Tek scopes I have used in the past had a "grounded" banana jack on the front panel which I plugged into for ESD management. My thinking was since it was industrial gear, Tek designs robust equipment, and the point was probably put there for that purpose, it was fine.
If you are only dealing with isolated portable gear, I think it is more important to equalize potential to the gear, rather than to ground. .I have clipped onto grounds of gear in these cases. In this case, some (all?) soldering ground tips are grounded, so beware when soldering.