Wow! Sorry to hear that, It never occurred to me. Dumb question: did you turn on the psu BEFORE turning on the computer?
Try rebooting only with your quadro installed and check your nvidia drivers are up to date with your 10.10.5.
When I updated to 10.10.4 (from 10.10.3) my screen stayed black forever also. In fact the compter booted, but nothing showed because the drivers were out of date. I used another computer to access it via remote desktop and installed new drivers.
But I never experienced electrical boot problems, even mixing various brands of various cards (both mac and pc) on various slots, with different external psus.
Mac Pro 3.1 hardware
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^^^ try this. I had some odd startup issues until after a few updates. Nvidia seemed to fix it. Boot to your other card, update, and retry. I had exactly the same issue recently (kernel panic on startup). The latest drivers for 10.10.5 fixed this.
Windows 10 - 64GB RAM - Cinema 4D R20 - RTX 2070 x3
Yeah, it was a driver issue! Installed the latest and now it boots! My Quadro runs the monitors and the 980 computes the heck out of things!
A comparable scene rendered in Octane using the Quadro and then the 980 sure looks better
But I'm most impressed with using TurbulenceFD! My god is the 980 fast!!
One thing with the driver though, I cant switch to the Nvidia web driver! It stays locked to the OS X native driver. If I click to change the driver, the Driver Manager asks for my password, and says that I need to restart to use the Web drivers. After the restart though, I'm still at the native driver. I asked in the macrumors forum, and someone told me to "Manually add nvda_drv=1 to your boot-args using terminal." I'm not sure how this works, has anyone tried doing this?
Thank you!
A comparable scene rendered in Octane using the Quadro and then the 980 sure looks better
But I'm most impressed with using TurbulenceFD! My god is the 980 fast!!One thing with the driver though, I cant switch to the Nvidia web driver! It stays locked to the OS X native driver. If I click to change the driver, the Driver Manager asks for my password, and says that I need to restart to use the Web drivers. After the restart though, I'm still at the native driver. I asked in the macrumors forum, and someone told me to "Manually add nvda_drv=1 to your boot-args using terminal." I'm not sure how this works, has anyone tried doing this?
Thank you!
MacPro 5.1, 2x3,33 6-core, 48gb ram, GTX980Ti (to run Octane), Quadro4000 (to run screens), os 10.10.5, Cinema 4D 16
Great news! That's a relief! Anyway, welcome in the Octane realm! 
Regarding your driver which doesn't want to load I don't know which (probably hidden) file has to be edited, sorry.
Regarding your driver which doesn't want to load I don't know which (probably hidden) file has to be edited, sorry.
MacPro 3,1 (early 2008) / 2 Xeon 4x2,8Ghz / 16GB RAM / 2 Titan X (1 Hybrid) / OSX 10.10.5 / Cinema 4D R17.048