If I install macOS Mojave or Catalina on an external HD, will this change my internal HD filesystem?

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Level 1

9 points

If I install macOS Mojave or Catalina on an external HD, will this change my internal HD filesystem?

Cheers to everybody.

I own an iMac Retina 5K 27-inch 2017 (iMac18,3), running macOS 12.12.x Sierra from its internal HFS+ 3 Tb Fusion Drive, but I will likely upgrade to macOS 10.13.x High Sierra in the next days. I am simply testing my “old” software just to see if it works fine under High Sierra, so just doing my homework. But I postponed from day one since I was afraid of changing the filesystem of my Fusion Drive which I want to keep in HFS+. After having checked that High Sierra installer would not change my internal Fusion Drive filesystem at all, I am fine with the upgrade after all.

Now, my strictly related question: should I ever wanted to test macOS 10.14 Mojave instead, but INSTALLING IT ON AN EXTERNAL HARD DRIVE, will its installer possibly change my internal Fusion Drive Filesystem from HFS+ to APFS (which is something that I definitely want to absolutely avoid doing)? I guess it will not, but I really want to be on the safe side before doing this, hence my question here to you guys and the community.

By the way, it might seem strange, but I cannot find any info googling around. It's likely a matter of searching with the correct words (thing that I am clearly incapable of doing, as it seems), but still… I am stuck without any answer. Hoping to receive useful ones from you.

Thank you very much indeed.

Posted on Jun 22, 2020 6:02 PM

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Answer 1

MacCekko Author

Level 1

9 points

Jun 22, 2020 7:30 PM in response to samtenor

Thank you Samtenor for your swift reply, but I still fail to understand: as per my previous post, do I risk something with the filesystem on my internal Fusion Drive, in installing, say, Mojave (or even Catalina, gulp!) on an external SSD/HDD? I am sorry for being dumb… but since the complete lack of 3rd party tools to check disks' integrity etcetera in APFS disks (I still completely rely on Alsoft's DiskWarrior, while I do not really adore Apple Disk Utility, actually), I cannot have my internal Fusion Drive changed in its filesystem or it would be a total and horrific nightmare for me. So I really, really need to be ultra-mega-super-hyper-sure about this. ;)

Thanks you. :)

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Answer 2

dialabrain

Level 10

125,735 points

Jun 22, 2020 7:47 PM in response to MacCekko

As long as you don't accidentally install Mojave on your internal drive it won't change anything.

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Answer 3

samtenor

Level 4

1,798 points

Jun 22, 2020 6:44 PM in response to MacCekko

You do not need to worry about this. I did this type of exploration on my 2012 MacMini Server to compare HFS+ and APFS (dual drives for dual boot - although not FusionDrive) - just for this purpose.

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