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Cannot see new bootable partition in Startup Di...

I have resized the main partition on the system drive of a MacPro5,1.  It contains OS 10.8.5.  I created and formatted a second partition (GUID) and installed 10.12.6.  It seems to perform perfectly well but I can only boot into it by holding Option on startup to give me a choice of systems.  It does not appear in the Startup Disk selection in System Preferences in 10.8.5.   Any idea why and how to resolve it? Thanks.

最后更新：2017-09-18 16:18:12

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