Ok one thing that folks don’t cover is that if you try to do a High Sierra upgrade (and probably any other upgrade), you get strange results if you have not complete FileVault Encryption.
One of our MacBook Pro’s was in the middle of doing encryption and then we tried to run an upgrade. The machine then hangs not being to run the installation. Finally, the fix was to boot into safe mode (hold the Shift key went booting).
Then you need kick to encryption back into gear and let it complete. It’s not obvious how to do this, but what you do is to click on the lock icon at the bottom of the System Preferences/Security & Privacy/FileVault and unlock it, this should cause the encryption to restart.
Then you wait the hours it takes for encryption to complete. Then while in safe mode, you can do the upgrade and then the next reboot all should be good 🙂
via FileVault stuck on “Encryption Paused” – Apple Community