More holy crap here. I found this answer and thought “Yeah, more BS on the internet , this will never work”. My Mac Pro 2013 was acting weird.. like Command +C would'nt work but both command and C worked independently. SO I thought maybe a reboot would fix it. But then nothing happen no boot noise, no ventilation. I disconnected everything and nobthing worked… Believe it or not after 20 minutes of despair this answer above the only solution. The computer rebooted after I indeed pressed shift while turning it on (nothing else connected except keyboard + thunderbolt 27 screen). Into Windows actually. Maybe this is aboot camp issue?