ASROCK H81 PRO BTC (R2.0) in Linux. PCIE slots do not detect GPUs

[wolfmyst]

I have bought other ASROCK motherboards and have rigs running successfully on them.
The first version of the H81 PRO BTC worked with the same graphics cards.
I bought two ASROCK H81 PRO BTC R2.0, and they're both presenting the same problem: PCIe slots do not recognize the GPU (regardless of the GPU or the PCIe slot used). The cards fail to be detected even if I try to install them directly onto the 16x slot without a riser.
I'm pretty sure the problem is not the GPUs because I have them running fine in other systems. The problem is present as early as startup, at BIOS, so before the OS (in this case Linux) is even loaded...which is why I think it's the motherboard. The system starts up with the integrated graphics only, as if there was no GPU connected. I have made sure the usual options in the BIOS were selected as I've done before with several machines: PCI Adapter as primary graphics, changing to Gen 2.0 etc. but unfortunately this did not solve the problem. The BIOS version is P1.2 which seems to be the only available one for this MB, I also tried flashing the BIOS with the same version regardless.. and nothing.

Have swapped all components CPU, RAM, GPUS etc.

Tried to contact ASROCK support, have not heard anything from them after 5 days.

I'm wondering if anybody is using this version of the H81 PRO BTC motherboard (R2.0) with Linux successfully and if they had to do anything in particular to make it work. I find it hard to believe that the two motherboards I bought just happen to be faulty.

[m1n1ngP4d4w4n]

You have to disable the onboard GPU otherwise it will not show anything on additionnal GPU's, i got this board and i had the same problem when i started with it.