This is a strange one: If you have a 27-inch iMac from late 2012 with a 3TB drive — yes, it looks like just that one specific model — you'll have to give up your Bootcamp to get Mohave.
From Apple Support:
If you see the alert 'Installation cannot proceed with Boot Camp configured'
If you have an iMac (27-inch, Late 2012) with a 3TB hard drive, you must remove its Boot Camp partition before you can install macOS Mojave 10.14. After you upgrade to macOS Mojave, you won't be able to use Boot Camp to install Windows on this Mac.
The alert "Installation cannot proceed with Boot Camp configured" appears only when you try to install macOS Mojave 10.14 on iMac (27-inch, Late 2012) that has a 3TB hard drive with an existing Boot Camp partition.
To install macOS Mojave on this iMac, first back up your Windows data, then use Boot Camp Assistant to remove the Boot Camp partition. After the Boot Camp partition is gone, you can install macOS Mojave.
If you're going through this, let me know. And if you have any inkling as to why it's so specific, my inbox is wide open.
We may earn a commission for purchases using our links. Learn more.