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'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.
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Rene Ritchie is one of the most respected Apple analysts in the business, reaching a combined audience of over 40 million readers a month. His YouTube channel, Vector, has over 90 thousand subscribers and 14 million views and his podcasts, including Debug, have been downloaded over 20 million times. He also regularly co-hosts MacBreak Weekly for the TWiT network and co-hosted CES Live! and Talk Mobile. Based in Montreal, Rene is a former director of product marketing, web developer, and graphic designer. He's authored several books and appeared on numerous television and radio segments to discuss Apple and the technology industry. When not working, he likes to cook, grapple, and spend time with his friends and family.