For my case, I am running macOS High Sierra 10.13.6 on a MacBook Pro 17-inch (early 2011). I also have a AirPort/Bluetooth board transplanted from a MacBook Pro 2012 model with modified system driver to support Continuity, so SIP is disabled (or rather only kext checking is disabled) on my system. I also had a failed update to Security Update 2019-001 leaving a "macOS Installer Data" folder, and my Mac automatically booting into the Update installer boot disk.
I restart my Mac holding the Option key to boot into my regular "Macintosh HD", visit "System Preferences" > "Startup Disk" to choose "Macintosh HD | macOS 10.13.6" and click Restart... button.
After it restarts automatically into my regular "Macintosh HD", I restart my Mac again holding the Command-R key to boot into the Recovery Mode; if I did not do the "Startup Disk" process above, I cannot seem to go into Recovery Mode properly. In Recovery Mode, I use "Disk Utility" to check and make sure my folders and files are intact, then use Terminal to disable SIP via the command "csrutil disable".
With another restart, I can now remove completely the "macOS Installer Data" folder from the root directory.
Download the "Security Update 2019-001 High Sierra" from Apple Support web site, and then run the update. After a while my Mac will restart, the Apple logo and progress bar will appear, and a short while later, the screen will go dark. Leave your Mac as it is for a couple of hours--I left mine overnight--as it is actually doing the update in the background. On newer system, the Apple logo and progress bar will actually remain on screen and countdown towards the end of the update installation--because of this behaviour and on a hunch, I left my Mac overnight to see if the update completes, and it did.