Dual-boot mid-2012 MacBookPro - High Sierra + Catalina?

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Dual-boot mid-2012 MacBookPro - High Sierra + Catalina?

I have a 15" mid-2012 MBPro that currently has OS X High Sierra 10.13.6 installed on the internal HDD drive formatted as Mac OS Extended Journaled. It runs perfectly fine.

The MBP is qualified for macOS Catalina 10.15 and I am considering installing Catalina on an external SSD to use with this MBP. I know that I need to format the external SSD as APFS and I have a bootable USB installer for Catalina.

If I install Catalina on the external SSD, will High Sierra remain bootable from the internal HDD ... or is there any risk that by installing Catalina on an external SSD that bootability into High Sierra will somehow be blocked or corrupted? (In this regard I am thinking of firmware updates that accompany Catalina installs.)

I want to end up with a dual-boot machine. High Sierra + Catalina.

Earlier Mac models

Posted on Oct 1, 2024 5:50 PM

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Answer 1

HWTech

Level 9

59,420 points

Oct 1, 2024 8:12 PM in response to MartinR

Installing Catalina to an external SSD should not affect your High Sierra installation on the internal drive. The system firmware update will still allow High Sierra to boot & work just fine.

However, you should make sure you have a good backup of your High Sierra installation first just to be safe. I hope you already have frequent & regular backups of the computer already.

The only possible issue is that you may lose access to the Apple Diagnostics....or if you already have lost access, then perhaps you may regain access to them with the system firmware update. I have seen some 2012 laptops lose access to the Apple Diagnostics, but others have no issues. I never investigated to determine which firmware versions had trouble. Otherwise I never experienced any issues with any of my organization's 2012 laptops. Losing access to the diagnostics is not a big deal since the diagnostics rarely detect any issues anyway, plus there are other ways of testing most of the hardware so don't worry about it.

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