Windows 11 does not see my 2nd GPU (4090 + 4080)

I have the following rig for AI:

  • Gigabyte 4090
  • MSI MAG X670E Tomahawk WiFI
  • 7800x3d
  • RM1000x — 1000 Watt 80 PLUS® Gold
  • 64 GB DDR5 @ 6000mhz
  • Fractal XL case

The motherboard has the following PCIe slots:

  • PCI_E1: PCIe 5.0 x16 (From CPU)
  • PCI_E2: PCIe 3.0 x1 (From X670 Chipset)
  • PCI_E3: PCIe 4.0 x4 (From CPU)
  • PCI_E4: PCIe 4.0 x2 (From X670 Chipset)

Here’s the manual: https://download.msi.com/archive/mnu_exe/mb/MAGX670ETOMAHAWKWIFI.pdf

I have a Gigabyte 4080 that’s sitting around collecting dust in another computer. I want to add the 4080 to my current rig to increase VRAM (16GB 4080 + 24 GB 4090 = 40GB) and hopefully token/second speeds.

The 4090 sits in the PCI_E1: PCIe 5.0 x16 (from CPU) slot on the MSI X670E MAG motherboard. It’s so fat that it takes up three slots. The only other available slot to mount the 4080 is the PCI_E4: PCIe 4.0 x2 (from X670 chipset).

I plugged the 4080 into the PCI_E4: PCIe 4.0 x2 (from X670 chipset) slot via a riser cable. I’m using an external Corsair 850W power supply to power the 4080 (and nothing else).

The problem is that Windows Device Manager does not recognize nor see the 4080.

I have already tried the following:

  • Swapped 4080 and 4090. The system does see the 4080. So, the card is good.
  • Changed the BIOS to x8+x8 (Windows still did not recognize it). Changed it again to x4+x4+x4+x4 and it did not see it.
  • Performed an hour of Google searching to no avail
  • Changed from “Auto” to “Gen3” in the BIOS
  • Ensure all cables/etc are plugged in properly.
  • Had the Device Manager search for new hardware (none found). Showed “hidden” items and the 4080 was not found.
  • A few other online “tips” that I found while browsing Reddit etc.

The only solution I can think of is to get a second PCIe riser cable and plug the 4090 into the PCI_E1: PCIe 5.0 x16 (from CPU) slot via riser cable. Then plug the 4080 into the PCI_E3: PCIe 4.0 x4 (From CPU) slot.

Thank you if you can help me figure this out.

Did you solved an issue? Maybe tried to put 4090 to PCI_E3 and 4080 to PCI_E1? I want to do same setup and need to know the truth :slight_smile:

Did you try changing the ACS and IOMMU settings in the BIOS (UEFI)?
Well, if Windows itself does not recognize the GPU, it might be more of a hardware issue…