Twice when reinstalling High Sierra or Mohave or updating either I have found that when I restart I have in my main directory a folder named PREVIOUS SYSTEM.
If you try to throw it away, you get an error message saying that CANNOT BE DELETED REQUIRED BY THE SYSTEM". If you use some disk management utility to try to force delete the folder you may well corrupt you drive. It has happened to me and a couple of others I have talked to.
There is a solution, thank goodness. Restart in Recovery Mode (COMMAND-R). In Recovery Mode access the TERMINAL and when it opens type in csrutil disable. This will disable the Startup Integrity Protection. Then type in Terminal reboot your Mac will restart. When the Finder opens drag the PREVIOUS SYSTEM FOLDER to the Trash and empty the Trash and the folder is gone. Finally restart in Recovery Mode…go to Terminal…enter csrutil enable. The Startup Integrity Protection will be instantly restored. Restart and you’re back in business.
I know that this solution works in new iMacs and Powerbooks using High Sierra and Mohave. I have no knowledge of other Macs.
Sure hopes this helps some of you! Two System Folders on your Mac can be disconcerting. Have a great day one and all!