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Question: 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.

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Nov 11, 2017 7:32 AM in response to uncle_sam473 In response to uncle_sam473

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.

Nov 11, 2017 7:32 AM

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Question: Booting from external hard drive