Q: El Cap won't dual boot Windows 8.1 w/ EFI
I have a 2009 Mac Pro. Upgraded the procs to dual x5690 and "flashed" it to read as a 5,1. Running a GTX 980 and Quadro K4000. 64 Gb ram. OSX installed on a owc pcie ssd, Windows installed on Samsung 850 pro plugged into spare sata connector in optical bay.
My quandary: I was running Yosemite and Windows 8.1 in harmony. Each one on its own drive, not two partitions of the same drive. I upgraded to El Cap. I could no longer boot into Windows if my El Cap drive was plugged in. I did some research and saw that El Cap doesn't like MBRs so I did a clean install of Windows after switching it over to EFI but it still won't boot. I use startup disk utility, and it reboots and just sits on a black screen.
Is there a way to get this sorted? Do I have to disable this new SIP feature that seems to be wreaking havoc on all sorts of folks? Help me Obi-Wan Kenobi....
Mac Pro, OS X El Capitan (10.11.1), Windows 8.1
Posted on Dec 11, 2015 9:51 AM






