I've been having this issue with a particular graphic card not being recognized by the octominer mainboard. It is an Asus RX580 Dual 4GB. I tried this card on 3 different motherboards all of different make/year/chipset MSI, Gigabyte, Asus.. It seems to be working very well with all of them. I even tried mining and some benchmarks with it, and it runs without issues. However whenever I attach it to the octominer it won't get recognized. It behaves like there is no graphic card at all. I tried adding only this card with no other GPUs, and it won't boot. It give 5 beeps signal(I think) which is for no GPU, that is with the onboard disabled. So it is not an OS issue, it simply won't boot. The interesting thing is that i have a bunch of all different kinds of GPUs on the octominer that all work perfectly well. All 8 cards on it currently are by different manufacturers. I have:

Zorac 1070
Palit 1070
KFA 1070

Sapphire Nitro+ 580
Powercolor 580
Gigabyte 580
XFX 580
Asus Strix 580

Do notice that an Asus Strix 580 works totally fine on the octominer. But an Asus Dual 580 will not get recognized. It's too bad that I don't have another of the same exact Asus Dual 580, but as I mentioned earlier I've ruled out the GPU being defective as it runs perfectly fine with other different kind of brand motherboards.

I also tried changing PEG Gen1,2,3 in BIOS with no avail. Tried a few more settings, PEG/PCI related that did not help.

Can someone please let me know what this could be? Is there anyone with a similar issue maybe?