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

[Max_Bvr]

If your problem is that your mobo doesn't recognize the gpus, maybe i have the solution.

Try to move your keyboard usb wire. I have a Alien keyboard with 2 usb, since I remove one of them, my mobo is recognizing all my gpus.

[R734]

If your problem is that your mobo doesn't recognize the gpus, maybe i have the solution.

Try to move your keyboard usb wire. I have a Alien keyboard with 2 usb, since I remove one of them, my mobo is recognizing all my gpus.

That's weird enough it might just work... I tried disconnecting my PS2 keyboard and USB mouse and still didn't get an POST, BUT the ethernet port was blinking differently so maybe there's something to it. Currently going through a power cycle to test that theory from a fresh state. 
Considering sacrificing a goat in a satanic ritual just to get all the odds on my side, as well.

[R734]

Ok, unplugging the mouse and keyboard didn't do jack. 

I'm getting more and more convinced there's a major issue with this board.
I tried booting up smOS via the onboard VGA after reconnecting my 4 GPUs and I was greeted by the "please install the PSU's 4-pin power cables to the 4-pin connections on your motherboard" screen DESPITE using powered risers AND having those 2 molex connected to the PSU... 
I ignored and booted anyway and the whole thing restarted after 10 minutes or so after a fan gradually started doing a funny noise. 

Also smOS couldn't seem to connect to the internet. 

I'm bummed. Big time.

[fullzero]

Ok, unplugging the mouse and keyboard didn't do jack. 

I'm getting more and more convinced there's a major issue with this board.
I tried booting up smOS via the onboard VGA after reconnecting my 4 GPUs and I was greeted by the "please install the PSU's 4-pin power cables to the 4-pin connections on your motherboard" screen DESPITE using powered risers AND having those 2 molex connected to the PSU... 
I ignored and booted anyway and the whole thing restarted after 10 minutes or so after a fan gradually started doing a funny noise. 

Also smOS couldn't seem to connect to the internet. 

I'm bummed. Big time.

Are you using a modular PSU? 

If you are: maybe you have accidentally attached the molex power cable to a sata slot instead of the one labeled molex or perif on the PSU end?

[R734]

Are you using a modular PSU? 

If you are: maybe you have accidentally attached the molex power cable to a sata slot instead of the one labeled molex or perif on the PSU end?

As a matter of fact I am. 
However I did double, triple check that I didn't make such a mistake. 

I have 4*2 slots labeled "6+2 PCI-E & 4+4 CPU" where I plugged the GPUs and CPU.
I connected the molex cable to one of the 3*2 slots labeled "peripheral & SATA", which is the only place where the cable provided would fit, anyway. 

I have no idea what might be wrong.
Thing is I got another mobo a couple days ago, an MSI z270a, and it can't seem to detect any GPU either (I tried with the 4 I already had and with one brand new out of the box).
Either my riser cables damaged the new motherboard (if that's even possible?) or the problem is my PSU (wouldn't know how to assess that) or I got yet another defect (not entirely implausible, I got it from a somewhat shady shop and the box wasn't even sealed anymore, I'm going back tomorrow to see what they might have to say about that).

[R734]

Ok, so I did test all my GPUs with a used mobo and my PSU and they all work, to my relief.

I RMA'd the 2 faulty mobos and I should get an H81 replacement this week. 

I'm obviously going to test the replacement without riser cables first to assess whether it works or not. 
However, if -like I'm now convinced- the riser actually fried the 2 previous mobos I'm not going to plug them in without somehow testing them beforehands.
I really don't know how, though.
I did examine them but didn't find anything fishy (apart from an apparent saving on solder).

Can I not measure voltages and such with a multimeter before actually endangering my new motherboard?
I can't find anything on the forums on that matter.

[mezerwi]

Im also having this problem.

The mobo can detect all 5 slot except gpu on the 16x slot.

the thing is, i got 3 board with same issue, and another one work flawlessly (16x detect the gpu).

Done the power cycle, gen 1, gen 2, onboard n pcie gpu.

No luck so far. anyone know what else should i try?