My Mac Pro 4,1 (flashed to a 5,1) that I just purchased yesterday has a Radeon HD 4870 was glitching so I grabbed my old gtx 660ti and plopped it into the bottom slot and booted it and no post so then I put both cards in and the Radeon had 4870 had a display but not the 660ti when I plugged in a monitor it did nothing. The primary operating system that I use is mac os Mojave and it didn't show any display so I got out my snow leopard HDD. And went to the system profiler and it shows up as display type: GPU but it doesn't show any picture when connected to another monitor.