External power supply + USB

I need more than 5V for part of my circuit but I want connect it to Arduino connected to computer via USB (for Serial). I have 2 sources of DC from mains - one with transformer and one switching (I guess). I am not sure if I can use them safely - I planned connect Arduino to USB so it will be powered from computer (powered from mains) and rest of the circuit powered from the external power supply. But I am afraid to connect grounds (or anything else). I think the computer is isolated from mains by it’s source so it is effectively floating as well as output of the transformer charger. But what about the switching source? I have read about ground loops. Is it real concern in such setup or is it just a nuissance distroting weak signals between devices far apart?

Thanks for help.

More detail on just what you need to do would help....

Grounding: Your computer chassis and USB ground should be connected to the building ground through the 3-wire power cord.

Your external power supplies should have a common ground point (usually the negative terminal of all supplies) that is connected to Arduino ground. All those devices should have their own ground wire to that common point.

The exception is if your externally powered devices are totally optically isolated from Arduino, like typical opto-isolated relays etc.

Currently I would like to try to program a GAL16V8. For this I need "high voltage" for programming. I could use battery + 5V or some DC/DC converting but since I want to do some standalone projects later anyway I would like to be able connect them to Arduino connected to computer for debugging. I know I could optoisolate everything but it would add much of complexity.

Anyway I am neatly sure 3 wires goes from the wall to laptop's charger but only 2 from the charger to laptop. So I think the laptop itself is isolated from mains and no connected to any ground. Both other chargers I use as sources have just 2 connectors - no ground wire at all.