Suggest regarding the Security Update for High Sierra. Manually download the update and Stop there. Perform a Time Machine Backup right away. Disconnect all external drives and restart the computer in Safe Mode - immediately at restart hold the Shift key and continue holding until the Apple Logo appear, then release. Safe Mode will do a Disk repair, clear system cache files and only load Apple Software, extensions and Fonts. It will load slowly - normal. Once logged in, it will look and perform differently - also normal.
Now run the Security Update located in the downloads folder on the dock and follow the prompts and it may restart several time before presenting the normal Log In screen.