I have verified now that it is a Windows/driver issue. I also tried SimpleminingOS and that recognizes the 6 GPUs without a problem.

I would prefer to mine in Windows so if anybody has a workaround, please let me know.

So I assume, that it is really an issue with the windows driver. I was able to get 5 GPUs detected by switching to PCIe Gen 2. The 6th is detected but unable to be properly installed (the yellow exclamation mark in the device manager). If I switch to Gen1 and try to start or auto (with miner mode enabled, so this is how it is supposed to work), the board immediately resets as soon as windows starts loading (even before seeing the desktop).

I have actually tried all possible combinations. So far no luck.

Someone else is having the same issue with a asRock H81 btc pro. Here and Here.
I also get the BSOD Attempted Execute of no execute memory.

I tried the atikmdag-patcher-1.4.5 already. Same result. Then I thought it might be an issue with the patch so I refreshed the BIOS on the cards. Even with all stock BIOS, it is not possible to boot with the newest drivers with 6 cards connected.

All the PCIe is set to Gen 1.

HI,
I think there is a driver issue in the newest driver 17.4.3. I cannot get the system to boot with 6 RX580s. I can mine with 4 just fine. I can boot with 5 but the fifth graphics card is not recognized correctly. I am using a Biostar tB250-BTC. On other forums (am I allowed to link here?) the same issue has been reported on different hardware.

Does anyone have a rig working with 6 RX5x0s? Or does anyone have an idea what I could try (I will probably try Ubuntu, if nothing else works).

I am having the same problem with the Mainboard. I think it might be a driver issue though. I can only use the newest driver, since I am using RX 580s.

@carloshell
Did you try earlier drivers (16.9.2). I think the Mainboards should work with them.