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If you have a 27-inch iMac (Late 2012) with a 3 TB hard drive, Apple says you must remove any Boot Camp partitions with Boot Camp Assistant before installing macOS 10.14 Mojave. Back up any important Windows data first, needless to say. Furthermore, after you update the Mac to Mojave, you cannot use Boot Camp to reinstall Windows. However, either Parallels Desktop or VMware Fusion should run on that iMac if you need access to Windows.
Apple offered no explanation for this extremely specific limitation. The Late 2012 model is the oldest iMac that can run Mojave, so it’s likely that the company felt it was better to support Mojave on it in general than to preserve Boot Camp on every configuration.