According to Apple KB, iMac (27-inch, Late 2012) with a 3TB hard drive will not support macOS Mojave upgrade until you remove Boot Camp partition.
If you want to continue using Windows on your Mac, import your Boot Camp partition to Parallels Desktop.
Check benefits of running Windows in Parallels Desktop vs. Boot Camp.
- Download and install Parallels Desktop 14 for Mac
- Import Boot Camp using instructions from KB 123314. All Windows files will be copied from Boot Camp to virtual machine.
- Remove Boot Camp partition using instructions from Apple KB HT204009
- Upgrade macOS to 10.14 Mojave.
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