Sajin
What happens when testing the last slot by itself with all the other video cards removed from the motherboard?
Note: If the last slot works fine with a single video card inserted into the slot, and with all the other video cards removed from the system then it's most likely a compatibility issue with the onboard plx chip. The slot is defective if a video card fails to work in the slot by itself with all the other video cards removed from the system.

Let me clarify. Reading back, I was not clear. I knew it right after I typed it. Of the 6 slots, only one is 1x8 PCIe. This is my culprit. Not "last slot by itself." I haven't thought it through enough, or haven't found a similar thread, so I don't know any sure-fire way to test that slot but to put a riser in it, and when I do that, then boot to BIOS, it shows it populated in the PCI configuration settings under Advanced. I suppose I need to get some other device besides a riser/video card to put in the 1x8 slot to check it.
 
Til you return, I'll DDU, install, reboot, repeat.
 
Erik Brown

Sajin
So you're saying #22 on the below image is the slot that is giving you the problem correct?

 
Correct. As said, I've tried various combinations of 1-6 cards, all cards are good. The system absolutely rocks with 5 GPUs going full load, flawless. But try to expand completely, and no go.
 
Erik
 

Sajin
Let me know how that goes.

The system will not boot with only one card connected through riser to Slot 5. Error Message A2. No other slots populated
shut down
Remove Slot 5 Riser/Card - then fully install GPU into Slot 1
reboot
The system boots normally, all functions check out
shut down
Add single Riser/Card GPU to Slot 5, now Slots 1 and 5 populated
reboot
The system boots normally, all functions check out, both cards will mine at maximum in Slot 1 and Slot 5 
 
From this point, I've tried a card in every slot and rebooted of course, but also just random and some not-so-random combinations. I had done this quite extensively already, but then it was recommended to update the BIOS, so I was at that step when you assisted. Almost every variance I tried rebooted with only the GPU in Slot 1 visible in Device Manager and operable. Interestingly, one configuration of Slots 1, 4, 5 and 6 would not see any graphics card at all. Nothing in Device Driver, not even onboard. Other GPU monitoring programs completely blank.
 
So it seems with all current settings, put anything in Slot 1 and Slot 5 and the others are disabled. And then some. I'm to the point I start considering enhanced consciousness so I can find the answer. Delve down possibilities. Probably more than 14 million scenarios. 
 
Thank you for moving the discussion and following up.
 
Erik

Sajin
Hmm. I bet your issue has something to do with the plx chip that is used on the board.

Would you surmise that it may be something faulty, or software?

 
Erik

Sajin
I'm thinking a compatibility issue.

 
 
OK, compatibility. Takes at least 2 things to be compatible, do you have any suspects for odd man out? Tracking the system back the way I have, building from the breadboard, every single piece functions as it should separately, just not all in unison if that 5th slot occupied and anything else besides Slot 1.
 
Playing with hwinfo64, it will see things perfectly with GPUs in Slots 1 and 5, and those 2 cards absolutely sing at their intended purpose when performing the duet. Make it a trio or more, and things get weird. Only 2 cards will show full correct information consistently (pic attached), Slot 1 all the time. Go past 3 cards and the black bean jumps around with no set pattern that I have been able to recreate yet. And the memory values are corrupted / wrong on all the other cards, showing negative values for memory, wrong card ID/brand  and other things you probably recognize as whack more than I do.
 
My thread with tech support has taken me the usual troubleshooting, and I relayed your PLX comment. They were questioning the amount of power necessary, and confirming I was using the  additional 6-pin PCIe power connector. I told them I was, and I had also been concerned about voltage stability, so I added a 2nd PSU. They also requested high-res pics of the CPU socket, to theorize about bent pins. I've attached those here, if it will allow me the privilege and also if not too large. They asked for the resolution but the system won't take anything bigger than 300KB or something.
 
Thanks again for all your attention. It's been a few days since I posted here, I was off cheating in other forums trying to get a crash course on PLX. Know any NFL history? We're in an injury timeout, I take back what I said about "5 is Fine." I think on next scheduled maintenance, Bledsoe's coming out, and we're going to give this kid Brady a shot.
 
That's my quick version of a joke some guy made to me (of me) on a foreign forum. I was trying to seriously plead my EVGA case, and the guy tells me he knows my problem and the solution. He sent me the picture attached. I'll send more of the actual board and screens later if allows this one through.
 
Cheers,
 
erikbrown