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Asus Maximus Ranger VII incompatible with Intel X520 SFP+ cards?

Level 7

So I have a strange situation with my desktop, to upgrade my home network I bought 2 Intel X520-DA2 network cards with SFP+ ports (one is Dell branded, the other is HP 560SFP+branded).

One for my desktop, one for my Unraid server and connected to a Ubiquiti UniFi USW-Pro-48 switch.

Anyway, upon installing the card in the PCIeX4_3 slot it works and I can install the drivers in Windows 10 etc. BUT as I also use the M.2 slot this PCIE slot falls back to PCI-E 2.0x2 speed which would still limit it to about 1 GBPS so no benefit compared to the onboard LAN port so I decided to install the card in the second red PCI-E 3.0 x8 slot but when I do that the system will not post or boot and the display on the motherboard goes to 40 which would indicate something with sleep/hibernate mode?

If I remove the card the PC boots fine, if I install it in the PCIeX4_3 slot it boots fine, if I plug it back in the second red slot (PCIE_X8_2) it wont post anymore and show code 40


Exactly the same with both networkcards.

Any suggestions? Or what could cause this incompatibility?


Other hardware in my PC:

Intel i7-4790
MSI Radeon R9 270 2GB GAMING (which is a PCI-E 3.0 x16 card, could this cause this as the PCIE_X16/X8_1 slot will revert to x8 speed when the second slot is in use and the card won't post at x8 speed?)
Corsair Vengeance 16GB RAM 
Samsung 970 PRO NVMe drive