USB port earthing

This seems like it should have been asked before but I can't find anything in the search.

I have 4 servos which require an external power supply to run. I am powering the arudino off the same power supply as it needs to have the same reference point for the PWM signal. The metal casing of of the USB port will be grounded when I plug it into a computer. Should I keep this isolated from the ground of the external power supply. The problem is complicated by the fact I may be connecting it to a laptop with no grounding.


The external power supplies negitave terminal needs to be connected to the arduino's ground, that is all.

Ok, thanks