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As written on MacRumors and other places Macs using 3TB Fusion Drives for the macOS and Boot Camp installations can't be upgraded to Mojave currently without losing the Boot Camp partition.

To fix this problem, Apple says that customers with the 2012 27-inch iMac with 3TB hard drive will need to completely remove the Boot Camp partition using Boot Camp Assistant before macOS Mojave can be installed.

After upgrading to macOS Mojave, Boot Camp will not be able to be used to install Windows on these machines. No other iMac models appear to be affected by this issue.

What is the technical reason for this?

PS: See also Apple knowledgebase document (which mentions only Late-2012 27-inch iMacs with 3TB fusion drives.)

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    Here's the Apple Support document that makes this assertion. Doing some Googling leads me to believe that it has to do with the size of the Fusion drive and past restrictions/problems using Bootcamp (see comments) with 2TB drives in general. I don't touch Windows if I can help it, so I have no real-world experience.
    – IconDaemon
    Commented Sep 27, 2018 at 12:44
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    I don't see how this question is opinion based. "What is the technical restriction blocking X from working?" The answer might be "because Apple's installer contains a completely arbitrary check to block X under Y circumstances for no apparent reason", but that's still not an opinion. (And given the above comment, it actually seems the answer is more nuanced/interesting than that.)
    – Wowfunhappy
    Commented Sep 27, 2018 at 19:11
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    I would suggest you post a new question regarding how to install Windows without using the Boot Camp Assistant. Include which version of Windows you wish to install, model/year of your Mac and that the macOS will be mojave.
    – David Anderson
    Commented Sep 27, 2018 at 20:09
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    I absolutely disagree with the close reason for this - there is a specific and quantifiable reason as to why this problem affects iMacs with 3TB fusion drives.
    – Allan
    Commented Sep 28, 2018 at 14:31
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    To (hopefully) close this discussion: The trigger for putting it on hold was the "Is a future workaround likely?" part in the original question. As soon as this was edited out the question started to collect reopen votes from reviewers. The flag helped to speed things up, thanks for that.
    – nohillside ♦
    Commented Oct 3, 2018 at 18:03

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The technical reason is that the fusion drive is an Apple engineered disk spanning software layer and Apple didn't implement / ship this for fusion drives on top of APFS. Someone has to write the actual code that runs behind that Boot Camp assistant program and the code was written to not start on that specific hardware.

Now - that decision was probably a business decision. We could speculate that it required tradeoffs or more engineering talent or failed some performance or stability test so they removed that capability technically during the planning of the OS before it was publicly released to beta in June 2018 and shipped in September 2018.

Boot Camp is three things:

1) Drivers to make Windows see the Mac hardware as legitimate
2) Scripts to repartition the drive.
3) Documentation and articles like the one you references to educate people how to use the software and what to expect.

Apple could have decided to leave these few Macs without the entirely new APFS and coded other exceptions, but I'm not at all surprised they just said - no bootcamp on some small subset of iMac that didn't ship with SSD as the main storage.

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