first aid high sierra volume could not be unmounted

It seems to have left behind a kext named .kext and once I deleted it, things work again. Before I knew this I installed Malwarebytes to check for spyware/adware causing the CPU usage, then uninstalled it after installing the OS X beta. I am using the Beta version because 10.13.2 had very high CPU usage by the WindowServer process that was fixed in 10.13.3. Update – I updated to 10.13.3 High Sierra Beta 5 and once again was unable to boot. Removed everything older than 2016 after running ls -lat. This time I had to remove kexts from /System/Library/Extensions. Update – I updated to 10.12.6 and once again was unable to boot. Update – This has been an issue for myself and others running OS X Sierra as well.

Update – It happened again when I updated to 10.11.4! Even fewer clues this time, additional details and an updated script to remove non-default kext files at the very bottom. Otherwise feel free to read the saga below!

If your OS X El Capitan/Sierra/High Sierra update won’t reboot you can skip to the fix or check here for the (mostly) automated fix script.

First aid high sierra volume could not be unmounted