Re: GPU 1 on Gigabyte Motherboard is always generating HW errors
Wow, you probably hit on of the eletronic / tech issues where there's no explanation 
You did a good job already trying most common problems, bios update, change risers, GPUs etc
You can try to change some settings on bios for sure
I have another motherboard, but I had some issues with 3 cards, the motherboard only recognized 2, no matter which positions, so I change settings on Bios, specifically on PCI-e section, and after some try and errors I found the issue.
You can try to change 1 parameter at time and see how it goes
Another thing to try, put a monitor or a adaptor on this GPU1
Try to clean this slot with isopropyl alcohol or a clean contact product
After that, I don't have more ideas, let's see if some person here have some idea
Or, you slot could be broken, some contact or pci lane is damaged
You did a good job already trying most common problems, bios update, change risers, GPUs etc
You can try to change some settings on bios for sure
I have another motherboard, but I had some issues with 3 cards, the motherboard only recognized 2, no matter which positions, so I change settings on Bios, specifically on PCI-e section, and after some try and errors I found the issue.
You can try to change 1 parameter at time and see how it goes
Another thing to try, put a monitor or a adaptor on this GPU1
Try to clean this slot with isopropyl alcohol or a clean contact product
After that, I don't have more ideas, let's see if some person here have some idea
Or, you slot could be broken, some contact or pci lane is damaged