Already done.It was my very first thing to do.
It seems to be reasoned by DDU.
I will share how I have done ASA I find the solution.
The solution is simple. First remove all gfx cards, all non essential HDDs and anything else attached to mothereboard battery to reset mobo, connect monitor to onboard hdmi and proceed
1) Reinstall battery
2) Boot (if some screen pops up about molex connectors ) hit 'n'
3) once you see bios screen , Go to advanced chip settings and make sure PCI-e versions are set to 'gen 1' and the preferred display out is PCI-e and not onboard GPU. Save the settings and shut down.
4) Use one gfx card , connect VGA/hdmi cable to it
5) attach the usb riser to the card and mobo 'on the first pcie x16 slot on H81BTC pro' and gfx card.
6) boot
7) Install gfx card drivers and shut down
now move the usb riser to first X1-pcie slot and boot.. Shutdown once you get the screen
9) Repeat the process on each of the pci-e 4 connectors .. one at a time (This way you know if some connector is bad too)
10) If all the pci-e connectors are able to show display and boot and let the drivers install.
11) Now that we know all pci-e connectors could identify gfx card , connect pci-e risers to other cards and connect all of them to all 6 pci-e slots.
12) Move the hdmi to gfx card connected to first pci-e x16 and boot.
13) all 6 gfx card should be seen now on the device manager.
14) You DO NOT need registry hack for 6 gfx cards in windows 10.
Let me know what happens