"I have the same issue when trying to reboot from Windows 10. The Option key during reboot works though. Since installing HSierra, when pressing Option at reboot, I have noticed that the Mac drive now shows up to the right of the Windows drive while before it was always the first drive from the left. Why is that?" - Mais78 Ive got exactly the same issue now but in 2020 with Mac OS Catalina 10.15.6 Was this ever fixed?