"Why Won't My Mac Pro Restart After the Latest Software Update?"
I have a 2010 Mac Pro 5,1, and after downloading a High Sierra security update from the App Store last night, trying to restart landed me on a black screen with a cursor, and that's all. I could power down by holding down the power button and then boot back up, but my next attempts to restart failed the same way. Checking online, I saw that the update included new EFI firmware, and that gave me a clue how to fix it. My boot volume was on a Sandisk SSD, but I had another system volume on an original Apple SSD meant for this Mac Pro. For past firmware updates to this machine, I'd had to boot into the Apple SSD volume to get the firmware installed. So, I tried that again, and sure enough, I was able to download the update and restart, with the update proceeding just fine. Unfortunately, this boot volume didn't have my latest files. So, back I went to my Sandisk SSD volume, used SuperDuper to copy all files to the Apple SSD volume, then booted back into that and updated it