Apple has published a support document warning that the iMac 27″ from late 2012 with a 3TB hard disk can’t use Boot Camp. It does not appear to affect other Late 2012 models.
During installation you will receive the following message:
Installation cannot proceed with Boot Camp configured
To install macOS Mojave on the iMac you should backup you Windows data and remove the Boot Camp partition.
Removing a Boot Camp partition
- Open the Boot Camp Assistant (in the Utilities folder).
- Select “Remove Windows 7 or later version,” then click Continue.
- Do one of the following:
- If your Mac has a single internal disk, click Restore.
- If your Mac has multiple internal disks, select the Windows disk, select “Restore disk to a single OS X partition,” then click Continue.
If you install macOS Mojave on this iMac, you won’t be able to use Boot Camp to install Windows.https://support.apple.com/en-us/HT209057