Problem: Only 3 (out of 5) GTX 1080 Ti GPUs Working

May 4, 2018
ASRock X299 OC Formula
Intel i9 7980XE (18-core)
GTX 1080 Ti (5x)

While I participated in the Hackintosh scene 10+ years ago, I've recently stepped back into it. This forum has been tremendously helpful thus far in building my customized system, thank you all for the support!

Problem - 1: I can't get the system to boot up when more than 3 GPUs are installed
Problem - 2: I can't get any GPU to work in the PCIe 1x slot on the MOBO. However, it appears that any of the PCIe 16x slots and the 4x slot work fine. I'm using risers for all of the GPUs.

Current Hardware:
- ASRock X299 OC Formula
- 5x GTX 1080Ti with Risers
- Intel i9 7980XE (18-core CPU) - Liquid Cooled with H115i PRO (Corsair)
- 32GB of DDR4 2666 RAM (Corsair)
- 240GB SATA Kingston SSD

The Primary Purposes of This Rig:
- 3D Rendering using VRay Standalone (Using CPU for this very particular task (I have a good reason for this, I swear), not GPU, hence the 7980XE)
- CUDA Accelerated Machine Learning

(All of the risers, GPUs, and other components have been tested individually to ensure that faulty hardware isn't a factor here. I'd assume that all of the issues and roadblocks I'm facing are software/configuration related at this point. I've been able to get this exact build fully functional when using Windows 10)

Thanks to a 6-hour Facetime call with a good friend of mine, we have everything else working on this fantastic build...but realized the limitation with the seemingly hardcoded 3x GPU limit implemented in High Sierra. I've seen some other users on this forum try some EFI fixes that helped them get over the hump, but the few that I have tried didn't work out for me. I'm newer to this, so I'm sure I am missing something relatively simple.

Ideally, I'd like to get all 5 of my 1080 Ti GPUs working, with the potential to eventually upgrade to the 7 or 8 that my motherboard technically supports. This would also require that I get the PCIe 1x slot working on the board, removing the 3x GPU limit in High Sierra, and eventually, and get the M.2 to work properly with my PCIe risers (though, the M.2 support is much lower on the priority chain for now and can be revisited later)

Any assistance or guidance would be greatly appreciated. Thanks again for all of your support thus far with your forums, buyer guides, and more. It's been tremendously helpful in getting things this far.

Thanks everyone.
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