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

60,320 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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Answer 2

MartinR Author

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Oct 3, 2024 4:03 PM in response to g_wolfman

g_wolfman wrote:

... you could just erase the main drive and make two APFS containers - or partition to an APFS partition and a HFS+ one, install Catalina, restore High Sierra from your clone, and have everything on the internal drive.

Not if the internal drive is a traditional hard drive. Mine is, and it's formatted as HFS+, which I said in my initial post. I do not want to risk disturbing the existing drive and its apps. I'll put Catalina on an external SSD & install 64-bit apps there. At some point in the future I may swap out the HDD for the SSD to have a completely Catalina MBPro, but not now.

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Answer 3

MartinR Author

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21,613 points

Oct 2, 2024 6:48 AM in response to HWTech

Thanks. I already have a clone of the HDD and keep multiple backups of all data.

Also, it's one of my secondary machines and is never the store of any critical data, so if the existing High Sierra installation goes south it would be annoying but not a crisis. I have some 32-bit apps that are the reason I want to keep High Sierra available but if I lose them I'll just have to move on. In that case I'd replace the HDD with the SSD and just run Catalina.

I plan for a new MBP once the M4's are released. But this older i7 MBP will still be my travel machine for the forseeable future.

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Answer 4

g_wolfman

Level 4

2,056 points

Oct 2, 2024 7:36 PM in response to MartinR

If you have a clone already - why go with an external drive (unless it's disk capacity?). Depending on whether your current High Sierra disk is APFS or HFS+, you could just erase the main drive and make two APFS containers - or partition to an APFS partition and a HFS+ one, install Catalina, restore High Sierra from your clone, and have everything on the internal drive.

I basically did that with my two current laptops (Ventura/Sierra and Catalina/El Capitan/Mountain Lion) - works perfectly fine.

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Answer 5

HWTech

Level 9

60,320 points

Oct 2, 2024 9:19 PM in response to g_wolfman

If @MartinR is using a hard drive for the internal drive, then using HFS+ is best since the APFS file system tends to add extra wear to a hard drive & will cause a performance impact.

If the internal drive is an SSD with sufficient Free space, then I would agree with converting the High Sierra volume to APFS and install Catalina to another APFS volume. I never recommend multiple partitions/Containers on any drive because people almost always realize at least one of the partitions is too small. The only exception would be if the drive is large enough and the user is never going to store large amounts of data on the partitioned drive.

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