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Since installing High Sierra I’ve had trouble booting into Mac OS X from Windows 10.

I use a 2016 MacBook Pro, run closed, hooked up to two external monitors.

When I restart Windows, I hold down the alt key to select the startup disk and select the Mac drive.

The next step is that I have to enter my password to unlock FileVault.

After the disk seems it unlock, the boot process stops on a black screen, then the fans go silent and the Mac stops responding. I’ve left for periods exceeding 15 minutes just to make sure it wasn’t thinking about booting.

If I force a restart by holding down the power button, the startup chime sounds twice in rapid succession, then boot up continues.

However, sometimes the external monitors don’t get a signal, but the MacBook screen works.

And sometimes - not always - I have to unplug a USB hub for the reboot to continue. (The hub has a keyboard, mouse and a couple of USB cables in it for charging random gadgets.)

Once started the Mac runs normally.

EDIT: At no time do any error messages appear.

Resetting the PRAM and SMC doesn’t help with this.

Any suggestions for a fix? It doesn’t seem to be a critical error, but it’s annoying.

If you need more details, please say so in a comment.

Thanks.

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    Just out of curiosity, what happens if you change the startup disk in Windows through the Boot Camp control panel? – PoisonNinja Apr 9 '18 at 17:16
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    @PoisonNinja With High Sierra, you can’t select the Mac’s boot drive as the startup disk from the Windows 10 Boot Camp control panel. It’s because the new disk system High Sierra uses is very much incompatible with Windows 10. – Richard Cosgrove Apr 9 '18 at 21:00
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    Ahh my bad. I just found support.apple.com/en-us/HT208123. What about safe mode? – PoisonNinja Apr 9 '18 at 21:38
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    @PoisonNinja Don’t worry: I think everyone got caught out by that. I’ll try safe mode next time I’m crossbooting and post. – Richard Cosgrove Apr 9 '18 at 21:44
  • @PoisonNinja Booting from Windows 10 to High Sierra led to black screens; a restart and it booted normally. Booting from W10 using safe mode boot was normal. – Richard Cosgrove Apr 11 '18 at 23:32

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