So it turns out these boards only like certain risers. Version 6 i believe. This is why my board only saw 3of6 gpu.

Swapping risers solved my mobo's identifying issues

I was under the impression that Microsoft used MBR by default on all windows installs. So why would i have an MBR issue to recognize additional cards.  

My AB350 board did 6 cards (not of the same brand), my asrock does 8+2 and now this Asus prime Z270-a only does 3 cards of any type.  All systems are running off a normal laymen's install of windows.

This drive was setup on another system and transferred to this mobo. It was running 10 1070 + 2 390s.  Do you think i should still try the mbr version?

Yes i did but that Should only affect memory mapping over 4gb and not Windows detecting the gpu....according to the list below

I'm going to try to disable sata ports as i don't use them.
Any other idea?

Fellas,

I  have an Asus prime Z270-a and it's supposed to run up to 8gpu, though I can't even get the 4th to work.

I have 3 GTX 1070 and 1 R9 390 to run on there. the r9 390 isn't showing up in device manager.  I've configured all my PCI-e links according to everyone video I've seen on the board, but the 4th card just isn't showing up.  This isn't my first build, though it's only my 3rd and my first time on Asus.

Yes, the bios is updated
Yes, I've tried 3 different Risers, though the first two are known to work.
Yes, I'm only plugging the risers card into PCI-E 1x slots.
Yes, the card works when plugged into the board with no other cards.

Thank you.

I cannot install AMD drivers as no device is recognized.

I didn't have 5 cards before. I had 4 cards running previously, now that I have 5 I was hoping to get them all to work. Even though window's sees 5, one has a with triangle on it.  The triangled card is was also working previously.

Driver's were reinstalled with DDU in safe more many times.

I also just installed Bios version 3 too.  I'd have to say that installing all of your cards to the mobo, running DDU, then doing a bios update was the easiest way i had to get these cards recognised without doing some monkey work.

Any other suggestions for this last GPU?

I also have this board, and I am able to run 4/5 gpus' on it.  I'm also running 2x hx 850 watt gold rated power supplies.

Working GPUs:
2x gtx 970
1x 1060
1x 1080

Recognised, but not working (yellow triangle)
1x gtx 970.

Before I had the 1080 put in, all gtx 970 were working.  Now I have the 5th card recognised by windows, though there is a yellow triangle and no explanation why this card is being disabled by windows.

Anyone have an ideas about have yellow triangle? I'm going to try a third riser on the board to see if i have 2 bad ones. Seems to me that if there was a bad riser that the card wouldn't recognise in windows.

I also learned that USB 3.0 extension cables do not work with pci-e risers.