Possible solution for iMac 2009 late no booting anymore
Hi, tried everything written and what i found about this problem and no result. Also a new hard disk, reinstall etc.
When I tried to fix with the disk utility, I got the same error every time. One of the volumes remained active and could not be put inactive. At that moment I realized that there could be a mistake in that. I then transferred the OSX volume to a USB stick and after booting from it I completely emptied the hard disk and then reinstalled High Sierra and no problem any more til today.
I don't know if it is THE solution, but it has turned out to be a way to solve the problem.
Good luck John.