G'day Rich,
I went and did a little more reading... I was mistaken, USB isn't actually a true current loop signal, it is an Asynchronous an/or Isochronous differential NRZI (Non Return To Zero Interface) encoding system using voltage reference between the D+ and D- wires. Now, while there is no actual ground reference in the data signal, the Data lines are tied to voltage sources that are referenced to "ground". If this "ground" is tied to the external ground somehow, then a ground loop is still possible, in a round bout fashion.

In addition, there is a 5V and ground extended up the USB cable for powering downstream devices.

So much for gettin' all excited there...


--=-- My credo: If it's worth doing, it's worth overdoing - just ask my missus, she'll tell ya laugh --=--
You're only paranoid if you're wrong!