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Booting from external hard drive

My MBP is on MacOS Sierra 10.12.6.  I tried to boot from my external hdd backup with OS X Yosemite (power on pressing option key).  When I choose the yosemite hdd backup, a blank space appears.  Seems it cannot run my yosemite backup, so it's in conflict with OS Sierra?  Need help here.  Appreciate the advice.

What type of backup is it? There should be no conflict with Sierra if it is on a separate volume.

However no Mac can run a system that pre-dates the OS the Mac originally shipped with. If that's the case it will never boot.

最後更新：2017-11-12 00:43:17

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