If you are using Bootc on your iMac, you may not want to upgrade to macOS Mojave. There is a specific configuration that Bootc can not execute and can not even have a Bootc partition on your iMac drive.
macOS Mojave does not support Bootc on a single iMac configuration
With Bootc you can boot your Mac with Microsoft Windows instead of macOS. Your Mac becomes a PC while Windows is running and you must restart in macOS to reuse your Mac apps.
This comes from Apple & # 39; s Knowledge Base:
If you have an iMac (27-inch, late 2012) with a 3 TB hard drive, you must remove the Boot C partition before you can install macOS Mojave 10.14. After you upgrade to macOS Mojave, you can not use Boot C to install Windows on this Mac.
macOS Mojave is the latest Mac operating system from Apple. It adds new features like tacks Desktop file organization, Quick Actions in Finder windows, enhanced screenshot and recording with built-in formatting and editing, system-wide Dark mode for apps, Continuity for grabbing and editing photos that you have on your iPhone cuts.
macOS Mojave is a free upgrade available through Apple's Mac App Store. If you use a 27-inch iMac from 2012 with a 3 TB hard drive and need Bootc, you can better add the cost of a virtualization tool such as Parallels.