2017-11-18 Sierra Security Update bricks 2012 MacBook Pro Retina

After choosing to run the new Security Update issued this morning for Sierra on my 15" 2012 MacBook Pro Retina, it never booted normally again. The update process seemed to never complete and I've been able several times to get the startup chord to play and to see the backlights of the keyboard, but the screen has, almost without exception, remained black. No gray apple at boot, nothing.

Attempting to boot into Firewire Target Disk Mode with the Thunderbolt to FW800 adapter and an 800 to 800 cable connected to my Mac Pro fails, again with nothing but a black screen. No external disk becomes visible to the Mac Pro.

Updating the NVRAM and the SMC of the MBP both seem to accomplish nothing. Booting into Safe Mode likewise.

Twice, while holding down the power button (and perhaps inadvertently the key right below it) I momentarily saw the login screen with two login choices side by side but it wouldn't remain visible for over 1 second and I couldn't get anywhere at that point e.g. entering my password to log in.

This is almost certainly a software issue, as it occurred precisely when the Security Update was done, but I've not been able to find a single reference to this happening searching on posts over the last 24 hours using a variety of search terms. With a black screen it's always very difficult to tell what the heck is going on.

Well... After about 45 minutes of not working, then taking the time to write the above, I just re-opened the MBP and voila, there was the logins screen, looking as though everything was fine, and the notice "You shut down your Mac because of a problem" (several times, actually). I had waited plenty long for processes to complete, but something had to have gone very wrong during the Security Update process. The App Store application says that it was successfully installed. Consider this a warning and a bug report both.

Closing (sleep) and re-opening the MBP twice have worked normally so far, so the storm seems to have passed.