This is common with driver issues. Windows and drivers will screw up at times when you move cards around. Try using DDU then shutdown, move the cards and try again.
Also, your PC must have an oversized PSU. Moving to to the x16 can cause more power draw as it starts up.
So my PC had 2 GPUS in it before (a 1070ti and a 960).
I noticed the 1070ti was not in a 16x PCIE slot so I decided to change the 1070ti to the main slot and plug the 960 into the 1x slot and the PC turned on but the monitors did not turn on and boot up. I took out the 960 so just the 1070ti was in and it booted up fine.
I got a second 1070ti out and plugged it into the second lot and it didn't boot up again.
Can anyone help with this?
I had two graphic cards before?

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