Could it have something to do with having had a Time Machine disk plugged in while trying to update?
[doublepost=1528158997][/doublepost]Never mind, that didn’t help. Now how do I check root or what not?
Reboot to your other OS (or boot from your backup drive), and navigate to /Volumes/[ROOT VOLUME]
For example, I have several installs. Currently installing Mojave now. My mac will look like this:
/Volumes/Macintosh\ HD (contain a working High Sierra install)
/Volumes/macOS\ Mojave\ beta
/Volumes/Install\ macOS\ Mojave\ beta
Reboot, hold option and select your other, working bootable partition. If you installed Mojave as the only bootable OS on that computer, boot to target disk mode and use another computer to explore the drive (
https://support.apple.com/en-au/HT201462).
If you do not have another computer, erase your mac, reinstall High Sierra and restore from your backup. Then try the update again.