How to switch between Windows and macOS in macOS High Sierra
If you use Boot Camp with macOS High Sierra, press and hold Option while restarting your Mac to choose an operating system.
macOS High Sierra upgrades solid-state drives to Apple File System (APFS). This system currently requires Startup Manager to switch between Windows and macOS. If you choose Restart in OS X from the Windows Boot Camp menu, this message appears: "Could not locate the OS X boot volume."
Follow these steps to boot into macOS or Windows:
Restart your Mac, then immediately hold down the Option key.
Release the Option key when you see the Startup Manager window.
Select your macOS or Windows startup disk, then click the arrow or press Return.
Repeat these steps any time you need to switch between operating systems.
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