USB power saftey

So I am always super carefull to never hook up more power sources than needed to my projects for lack of understanding exactly how a particular power source is wired. However this is getting more and more inconvenient. What I mean is if I am doing an arduino project, and I have it hooked up to an extrnal power supply (benchtop etc) I ONLY upload code using my laptop running on battery power. I also do not connect my scope to the circuit if it is hoooked to anythign that goes to the wall, just in case.

however like I said, this is now super inconveninet as I am doing a LOT of testing w/ some stuff that needs a bit more power than my 12 V battery can supply so I am using a wal wart. But I am using my desktop and don't want to connect/disconnect all day during testing, since sometimes I am uploading new code every 45 seconds while trying to tune some stuff. Biggest concern is toasting my PC. It is only a year old and cost me a good 400 bucks (yeah I am cheap, but its better than the P4 it replaced)

Is my concern of using a wall wart and a pc on the same arduino at the same time legitimate? None of my wall warts have an earth ground, so everything I understand about electronics says I am safe, but I am not ready to risk it when the price of being wrong is my PC. And what about power sources that do have an earth pin? err on the side of saftey, or if there is decent continuity from gnd on powersupply 1 and gnd on power supply 2 (<100k or so) can I just plug it in and go for it?

My big concern is my usb 5v on the computer being 5v ref to earth, and for some bizzare reason my second power source (converted desktop PSU, second computer, wall wart w/ earthing etc) have gnd be at 20 volts and 5 at 25, relative to earth.

You can safely connect everything together.
For extra safety you could add a usb hub between the laptop and the Arduino.

The ground clip of an oscilloscope is usually connected to the ground of the mains. Connect that ground clip to the GND of the Arduino. The laptop and the wall wart are not connected to the mains ground. So even if the ground clip of the oscilloscope is accidently connected to something else, there is still no problem.