Installation of macOS High Sierra Update 10.13.4 failed; how do I recover without losing data? This morning I updated macOS High Sierra from 10.13.3 to 10.13.4. It appeared to install fine (I got to the login window), so I clicked on "Shutdown", and when I got to the office, and turned on my Mac, it rebooted into recovery, with the Installer Log open, and a dialog that read, "The macOS Installation couldn't be completed".

I called Apple and they had me run a check disk, and the boot-up volume didn't have any error. My internal SSD showed up just fine, unlike what happened to this poor chap. Apple also had me try to restart in Safe Mode, but the Apple icon got stuck at 100% progress (3+ minutes waiting).

So, since my drive appears to be intact (I used Terminal to browse my data), is it possible to undo the update or reinstall macOS High Sierra and have the system work as it was before the update (apps and data still accessible)?

Note, I use Time Machine backups, and I have a 2TB drive with about 40% free space. There are lots of "snapshots" on the volume. Is there a way to recover the system to the Time Machine snapshot taken before each update? Or, will I need to reinstall macOS High Sierra?

Since posting this, Apple called me back and they said that I should be able to reinstall macOS Sierra without formatting the drive and I should be back to the state I was before the failure. I haven't tried this yet as I wanted to see if others have followed this or another procedure to recover from this failure without blowing up the hard drive and starting from scratch.
Fortunately I was able to simply reinstall macOS High Sierra by booting into Recovery Mode (Command ⌘ + R) and choosing Reinstall macOS High Sierra.

I did not erase my drive first since system volume was intact (no errors after running Disk Utility). I simply reinstalled the OS and I was able to log back in with all my data, settings, and apps still there just like before. 

Also, this installed the 10.3.4 version of macOS High Sierra, so I don't have to go through the update process again.