Not getting video from GTX 970, Mac Pro 2008 (MacPro3,1) We had a hard drive crash with 10.10.5. I replaced the hard drive, reloaded 10.10 using my native ATI 2400 XT, ran all updates, installed latest web drivers, added the nvda_drv=1 boot argument, shut down, popped in the GTX 970, heard the chime for start-up but did not get to desktop. I am aware that the boot screen won't appear. I put the GTX 970 in a PC and got video. What could be the issue?
This was solved by:


making sure the PCIe cables were properly seated, on the Mac Pro motherboard.
clearing NVRAM by cmd + option + p + r.
installing Nvidia drivers (which adds the nvda_drv=1 flag as part of the installer).
place GTX 970 in slot 2, making sure PCIe connection is on tight.
system boots properly.
shutdown, then move card to slot 1, making sure PCIe power connection is on tight.
system boots properly.