I've been having a problem with Security Update 2019-004 10.13.6. When it was time to restart to complete the update, the screen went black and the mouse cursor moved to the top left corner (I could still move it using the mouse). After a while it was obvious it was stuck, and the only way out was to power down using the power button.
I've also tried installing it in Safe Mode, no luck. I also tried Internet Recovery and the problem persisted.
Whether I try to restart or shutdown, same problem.
Surely there's an issue with whatever program or script whose job it is to install the update. I've done quite a bit of "googling" and cannot find where/what it is and how to remove it so that it won't try to execute. If I can't find that, the only way to ever be able to restart/shutdown properly ever again might be a clean install, which I'd rather not do (lots of stuff configured on this iMac...)
The machine is a mid-2011 iMac running High Sierra.