I'm pretty sure my second PCI-E slot is dead, so I want to try to SLI with the first PCI-E 16x and the 3rd which I can set to run at 4x. I read that I will attain 95% of the performance of using 16x/16x.
Anyways, I just cannot enable SLI when in 16x/4x mode. Both cards shop up in device manager, I can use the second card for physx, but I can never SLI. Tried older drivers, clean reinstall. Nothing works. No troubleshooting help from the manual neither.
SLI requires that all slots that are being used be at least x8. The driver will never let you enable SLI with one of the cards in a x4 slot. That's Nvidia's decision, not a design limitation (obviously, since Crossfire works just fine with a card in a x4 slot).
Edit: I do remember seeing something about a SLI hack, but it's old and may or may not work with newer hardware and drivers.
Yeah, I've seen that before and I honestly don't know how they did it.
It's possible that if a board is SLI certified to begin with, using a card in the x4 slot would work, but then, it should be working for you. It's more likely that the drivers allowed it then (and they don't now) or they used the SLI hack.