For me I tried everything including the Options key, Options + R and even remounting hard drives in Windows Registry Editor but what worked in the end was simple and a few other people have suggested it. In Windows just update the Apple Software Centre. As easy as typing in Apple in the windows bottom left search bar, opening Apple Software Centre and clicking update. This should update Bootcamp to the latest version and then restart as it asks you to do and then voila, you then open bootcamp in Windows and you'll see the Mac OS partition to boot from again. Phew!
ScotzUK
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