I have a iMac retina and was running High Sierra. This morning I installed the latest security update. It installed successfully and the computer rebooted. A short while late I rebooted the computer for another reason and the computer failed to boot successfully. A macOS installation screen came up and I was told that the "Volume contains a mac OS or OS X installation which may be damaged." I restarted the computer into Restore mode and ran Disk Utility. The drive checks out as ok. I tried restoring from a Time Machine backup - it failed. Same message. Tried with 3 different TM backups - all failed. Tried to re-install High Sierra over the Internet - it failed. Tried to install Mojave over the internet - it failed. Got out my USB drive with a week old backup and booted from that drive. The computer boots no problem. Ran Disk Utility on the iMac HD. No problems. I find very little info on the web about this problem, none of it applies specifically to my problem. Any help/ideas would be appreciated.