help me find what connection in pico killed my usb hub

Hi all, need some help with pi pico. I followed a tutorial to get a 4 digit 7 segment display work with pico, and it worked absolutely fine, I was able to see the display working properly. (https://how2electronics.com/using-7-segment-display-tm1637-with-raspberry-pi-pico/) But then I wanted to find how much current was consumed by the display, so I did two experiments (in both cases pico is connected to macbook pro via usb hub): 1) I connected the display to the pico and measured the current by connecting the multimeter in series with the pico(pin 38) and the ground pin of the display. (I did not remember exactly, but the display did not turn on I guess) 2) Did the same experiment as above, but this time I connected the multimeter in series with the pico(pin 36) and the vcc pin of the display, when I did this, the usb hub that I had connected to use the pico with macbook just stopped working forever. but the pico itself just works fine. I felt very bad for what I did, lost my usb hub. But still unable to understand what went wrong, pls explain to me what happened and how to avoid it in the future. Attaching pictures of my second experiment.

You should NEVER measure the current on the low side (GND) as the current can flow through the GPIO pins.

Measure the current on the +/Vcc side next time.

what is the issue with that? current flows in a loop right, I though we can measure anywhere.

that is what I have done in my second experiment, and it fried the hub, any idea why that happened ?

If the GPIO pin wasn't connected, then you are corrected?

sry did not get you question, I'm new to this. I have connected using 4 pins - ground, vcc, 2 gpios

Well then I will explain it like this. Your assumption is that the Pico reacts as a resistor and that the GPIO cant transport current. The fact is that the GPIO pins isn't protected for backflow, that would limit them to only some functionality and they are not. This is the reason for putting the On/Off switch on the high side of the circuit, NOT on the low side.

But why your USB hub stopped working is a mystery. It could have a explanation if you powered the pico with another power source but that is not the case. So my bet would be on a fluke, electronics breaks as all things can do.

how do I make sure I do not exceed the power limit of my hub hereafter? I dont want to risk these again. So I wanted to only copy the program from computer to pico and never run anything in pico when computer is connected. How can i achieve that? currently when I connect pico the the hub, it just starts running the main.py I wanted to copy programs only from computer, once done I want to disconnect from hub and connect a external wall plug to the pico and run it. how can this be achieved?

<@796000224690307072> sry for tag but any reply would help

I see it as a coincidence that the hub broke at the same time as you did the measurements. As you describe, you didn't introduce any power source just the chain MacBook - HUB - Pico - LED display and then you added a voltmeter between the display and the Pico. So it is a black swan moment.

To avoid any further risk I want to use laptop only for copying programs. I want to run programs only when connected to wall power. How can this be achieved?