Booting from mac operating system from a external hd

Spoony0313

Reputable
Mar 30, 2014
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4,510
My sister broke her macbook pro and she got the hard drive inside and bought a external enclosure which uses an USB 3.0.

She connected to her mac desktop and she wanted to get in her old hard drives OS. The OS is already in her macbook hard drive and she wants it to boot from it instead of the mac desktop hard drive.

How do you do this?