I am in a studio with several Mac Pro towers (5,1 models); we're upgrading all the GPUs to Nvidia 1080 cards because we're using Octane for 3D rendering. I've successfully gotten cards installed in 4 different Mac Pros (all 5,1 models, various CPUs). The cards are either Nvidia Founders Edition 1080 or EVGA GeForce GTX 1080. However, I cannot get cards to be recognized in 2 remaining Mac Pros. To be sure that I didn't have some software gremlin standing in the way (after many failed attempts at getting it to show up), I completely erased the OS hard drive and installed a fresh copy of 10.3.4. I then ran the security update, so the macs are all running 10.13.4 (17E202), with the Nvidia Web Driver 3220.127.116.11.107 and CUDA driver 396.64. Even still, when I drop the Nvidia card in the PCI slot and start up, I never get a screen. I use Remote Desktop to login and see that no graphics card is recognized. I've even swapped the hard drives between a Mac Pro that DOES work with an Nvidia card with one that's NOT working, and the problem remains - that Mac Pro will still not show a picture or recognize that an Nvidia card is installed in the PCI slot. Anyone have any ideas what I might be missing, or what could have gone wrong in 2 of these machines? I'm totally stumped and have not been able to figure out a solution. Thanks!