I have a little problem with grounding.
I realized a circuit which is controlled by the GPIO-Pins of the Raspberry. The function of my circuit is irrelevant. You just have to know that I need the 3.3 V and 5 V from the Pins for controlling some IC's. My Output needs 500mA (USB) so I need an external supply. I get this from the USB-Port of my PC.
When I connect 5V, 3.3V, GND from the Raspberry and 5V, GND from USB-Port it doesn't work. The Raspberry crashes. When I disconnect one Ground everything works fine.
I measure the two Grounds and they have a difference of 0.2 V to each other.
What should I do? Can I just disconnect the GND of the GPIO-Pins or can the Raspberry get damaged of it? What is the reason for such a different Ground?
EDIT: A little illustration:
The circuit is a sort of USB-Switch. With the GPIO-Pins you can choose which Device should work. So the MUX's are controlled by GPIO-Pins and powered by GPIO-Pin 1 and 2. USB out needs the 5V/500mA of the USB-Host. The Pins are protected with http://www.thebox.myzen.co.uk/Raspberry/Breakout.html
Do you need further information?