Going back to your earlier post. The part we don’t comprehend easily, or even recognize as a problem in the first place is ground loops and electrical resonances/leaks between two devices which have two separate power supplies.
This is a problem when you have two totally different designs of power supplies. A laptop power system with battery and an AC powered USB disk. Pretty different I’d say. You can get back EMF going and all sorts of polarity reversals as one supply shuts down earlier than the other.
Also understand that the typical PC box power supply is all too happy to kill the power positively and solidly when AC input conditions are not “right”. This is a good thing. It is not the same for AC powered USB disks. These things can linger on and keep-trying to work in a brownout condition. And there are times when the USB cable itself can become part of the power circuit.
