iMac says that macOS High Sierra is installed but is still running Mavericks

CLICK ON LAUNCH PAD. Then you should see an application icon to install MacOS High Sierra. Use that to continue your unfinished OS upgrade.

That is very strange. This may sound extreme but not sure there is another way.

Use the installer with a program called Diskmaker X and a (minimum) 8GB thumb drive. Once the installer has created the bootable thumb drive, eject it and back up your Mac. If you can make a disk image or clone it to another external drive, all the better.

Once that is done insert the thumb drive and reboot the Mac holding down the Option key. Soon you will get to the boot drive selection screen, select the installer and let it boot. It will take a while longer than you think it should.

When you get to the installer screen where you have a menu bar, find disk utility, launch it and select the partition that you tried to upgrade and format it. Once that is done (should only take a minute or less) exit out of disk utility and proceed with the installation.

You have just done a clean install of macOS. Once that is done, boot your system and create a user with a different username than the one you used on your backup. Log in and apply any updates that the App Store shows as available.

Once that is done run the Migration Utility (/Applications/Utilities) and connect the backup drive and mount the disk image that contains the backup (if that is the way it was done.

You can now use the Migration Assistant to move all your files and applications and settings back to your Mac.

You can then log out of the account you created and log in with the username and password you used previously. You can delete that account or just leave it there for troubleshooting purposes.

Yeah, this is a pain and it will take a couple (or more) hours to do, but should guarantee you get a solid (upgraded) system and your environment (with apps and files) back.

Do you happen to have another bootable drive attached to your system (e.g. a bootable backup)? If so, then it may be that you did install the new OS to your regular drive, but when you reboot your system it can't (for whatever reason) boot from it. In which case, the system will attempt to boot from the next bootable drive it can find. Which would be a drive with your old OS.

If the installer can't create a recovery partition on that drive (e.g. not enough space), then it will fail. Which might explain why the installer is failing on your second attempt to install the new OS.

What I learnt today:

• Clicking Update next to High Sierra in App Store only downloads the installer to Applications (even though it suggests the update was installed)
• If you don't have 8 GB of disk space remaining after the installer (~5.23 GB) is downloaded, it is immediately deleted with no warning.

So you need at least 13.23 GB of free space before you click Update in the App Store, or else you get stuck in this "updated but not updated" situation.