This should have been easy.
In my photo/video computer I have a GIGABYTE GA-Z97X-UD3H rev 1.1 motherboard with Windows 10. I've been running a Gigabyte Windforce 960 Nvidia 2GB video card, using it for Adobe Premiere Pro and Photoshop.
I replaced the 2GB card in the first PCI 16x slot with a nearly identical Gigabyte Windforce 960 Nvidia 4GB card, and that worked fine.
Then I tried to add the original Gigabyte Windforce 960 Nvidia 2GB video card as a second card in the PCI 8x slot...which should have installed close to transparently with just a bit of display juggling (I've had two cards running in systems before, but not this motherboard and these cards or Windows 10).
But I'm getting some weird faults. The system can't consistently find the OS on startup--it's not booting from C:
So I go into the motherboard set up and re-set the boot drive...which is mysteriously not the 0 drive as it should be.
No luck. OS is not found.
So I disconnect the power from my other drives, leaving only the 0 drive powered.
I go back into the motherboard set up...and again, it's moved away from the 0 drive. But with only the 0 drive powered, it finds the OS.
If I remove the second video card, everything is fine. Put the second video card back in--the motherboard cannot find the 0 drive to boot unless it's the only drive available.
BTW, the Adobe apps explicitly don't work with SLI, so I do not want the cards bridged.