I have a Radeon R7 370 as a Primary GPU (I know it's not even close to being a current card, but it's all I have). As that GPU only supports two Displays using "legacy" Connections (HDML, DVI or VGA), I tried to connect an even older GPU I found lying around (a Radeon HD 4670). The Intention was to have the R7 370 drive two displays (one HDMI and one DVI) and have the HD 4670 drive the third display (1280x768 VGA).
My Problem is: When the HD 4670 is the only GPU in the system, It does work (only the third Monitor was connected to it), but as soon as I put the R7 370 back in the system. the HD 4670 no longer does anything (okay, that's not strictly true, its fan does spin up at POST, but slows back down immediately after.)
Both cards are recognized in windows device manager.
I also tried to boot into Zorin OS 12.4 (a derivative of Ubuntu), where both card did work simultaneously.
Therefor I know it's not h hardware issue, but a pure Software problem.
On Zorin I haven't installed any AMD drivers at all; on Windows I currently have Radeon Software 20.4.1 installed.
I am aware the HD 4670 isn't supported in the current drivers, but as it works as intended as soon as I remove the R7 370 (whith the Radeon Software 20.4.1 still installed) and given it works perfectly on Zorin I don't think that should be a problem.
I should probably mention that only the third monitor (the one hooked up to the HD 4670) isn't showing any output, the other two (hooked up to the R7 370) are working absolutely fine.
GPU-Z states Crossfire is disabled (I don't even think it should be possible to enable it and I want to have it disabled anyway)
but FurMark stated AMD Crossfire is enabled.
I can't find any option in the AMD Software to disable Crossfire (should it even be enabled).
I am not trying to do any gaming with this System, and the HD 4670 is supposed to be used more as a video adapter than a Graphics card.
Any Help would be greatly appreciated.
Here's the specs of the system:
Ryzen 7 3700X
2x16GB DDR4 2666MHz
PowerColor Radeon R7 370 PCS+
Sapphire Radeon HD 4670 <- this is the troublemaker
Asus Prime X570-Pro
A PCIe Bluetooth/Wifi card, a PCIe Ethernet card, and a PCIe RS323 Serial Card. (which have all been temporarily removed to see whether they interfere with the problem here, and the don't)
Corsair RM 750x