How does grounding work if I cut the 5V pads or connect schottsky diodes while keeping the USB plugged in?

@Nominal Animal I think you are complicating/confusing what @live424 needs/wants.

I am sure that he is new to Teensy and electronics and has become confused by all the complexities.

If he can come back to us answering Pauls question in #18 I am sure that we can help him, but it does not help when we bamboozle him with technicalities which are unnecessary and over his head.

Sorry @live424 is I am putting you down I don't mean to. We have all been where you are at the moment!
I wanted to have data send to my computer but also power the teensy externally. I already solved it by following @nominals first reply and getting an isolator + soldering on two schottky diodes. But I still did not fully understand when a solution like this is necessary and when it is not. Now, I have more of an understanding of why and when I need to take into consideration the different grounding potentials of multiple power supplies which will help me with projects in the future.
Idk if people care, but leaving this here in case anyone wants to do something similar in the future.

I used the following product which isolates the tx and rx side of the usb and allows me to use an external power supply, send data to my computer, and extend the cable up to allegedly 165 feet at usb 2.0 high speeds but i only needed 50.