Installing Ubuntu on Mac Pro (OSX 10.11.6) with NVIDIA GPU:

Dear Mac Pro Experts,

PROBLEM: I’v installed Ubuntu 16.04.4 on Mac Pro 4,1 (early 2009) running El Capitan (OSX 10.11.6) with apple original GPU (NVIDIA GeForce GT 120 512 MB) and every thing worked perfectly. However, when I upgraded my GPU to NVIDIA GeForce GTX 970, installed the drivers, and turned on the system again it went to black screen.

APPLIED PROCEDURE: I followed the instructions here (https://www.lifewire.com/dual-boot-linux-and-mac-os-4125733) to install Ubuntu on my mac.

TRIED: I’ve tested many online resources for example her: https://www.rodsbooks.com/refind/bootcoup.html#osx but no success so far.

QUESTION: Any suggestions on how to resolve this would be appreciated, thanks.

SYSTEM INFORMATION (just in case):

Model Name: Mac Pro

Model Identifier: MacPro4,1

Processor Name: Quad-Core Intel Xeon

Processor Speed: 2,66 GHz

Number of Processors: 2

Total Number of Cores: 8

L2 Cache (per Core): 256 KB

L3 Cache (per Processor): 8 MB

Memory: 48 GB

Processor Interconnect Speed: 6,4 GT/s

Boot ROM Version: MP41.0081.B07

SMC Version (system): 1.39f5

SMC Version (processor tray): 1.39f5

Serial Number (system): CK***20H

Serial Number (processor tray): J5***XZA

Hardware UUID: 7BA18EAE-2132-59C4-B418-270CB0826236