I'm in a bit of a predicament. Once upon a time, I installed rEFInd on my 2012 Mac Pro. I eventually removed it.
I believe I also once tried to install Kali Linux in a dual boot environment.
Fast forward and I have tried to follow the steps in this guide to make a Windows-bootable external drive.
My final (and fatal) step was selecting the Windows disk in "Startup Disk" in System Preferences like so:
...except it said "Untitled" instead of BOOTCAMP. Also not to confuse you, I am running 10.12.3 and did not install Bootcamp.
Upon restarting, my Mac Pro passes POST and chimes, but boots to a white/gray screen with NO logo, and then proceeds to run some sort of ISOLINUX bootloader that then presents me with the Kali Linux efi splash screen that does not respond to input:
That was very unexpected, so I shut it down, pulled all the hard drives, every last one, and rebooted. Same behavior.
I'm a little worried, because clearly something has been written to the NVRAM onboard, and it is not responding to my Cmd + Opt + P + R
Also the Mac bootloader is not showing up. It does not respond to any keyboard input. Holding Opt does nothing.
How do I get out of this?