I came home the other day to a dark gray screen with only a cursor. I leave my Mac Pro on during the days so I figured it just had a hiccup and needed a reboot. After trying multiple times, I couldn't get past the startup progress bar. Apple logo shows fine, progress bar starts loading... then black/gray screen with only a cursor. After multiple trouble shooting steps (First Aid, PRAM, SMC reset, Disk Warrior etc.) I just gave up and restored from my clone. Anyway fast forward a couple of days and I noticed a "Security update" waiting for me (2016-003). I'm still running El Capitan. I clicked update, rebooted, and poof! Same symptoms. I suspect the first instance of the problem was the update too, but I never rebooted or noticed until I came home later that day. Frustrated, I researched the problem (now with a culprit in mind) and found someone with the IDENTICAL problem 4,1 MacPro flashed to 5,1 3.46 Westmere 6-core installed (mine is the W3690 vs. his dual processor X5690) http://apple.stackexchange.com/questions/264863/security-update-2016-003-10-11-6-issues Sounds like Apple bricked my pseudo-hackintosh. Has anyone else had this problem or found a workaround?