u/Hot-East-7084

I had a problem where my SD card wouldn't init on a breadboard. After several tries, I found that communication errors disappeared if I just touched the backside of the PCB (behind the SMD parts) with my finger.

So, I stuck some copper tape to the back of the module as a GND shield. But it didn't work. Then, I tried pulling up the shield to VCC via a 10k resistor, and now it works perfectly.

Can someone explain this? Why does a VCC-biased shield work when GND fails?