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I would like to create this partitioning on my mac.

  1. A partition A with MacOS version X.
  2. A second partition B with MacOS version Y.
  3. A third partition C with shared data.

I would like to make partition C accessible from A and B.

And ideally, since I also would like to share user configuration data and user data in general, I would like home folders to be in partition C.

This would mean that some users would be users of both MacOSes, or even if they are treated as different users, they would have the same home folders in partition C with similar permissions to read/write/execute files in those folders.

A benefit of this would be to minimize the space used for user data by using a common space instead of duplicating much data which would be the same for both MacOSes.

Is this possible technically?

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    Technically this is possible, you may want to make there the numerical user IDs are the same on both macOS versions. The bigger challenge probably are file and config formats changing between OS versions, especially in ~/Library.
    – nohillside
    Jun 6, 2021 at 12:30
  • Thanks @nohillside so would you do this if you were in my shoes? Jun 6, 2021 at 13:12
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    Share data only, not configurations :-) What is the focus of the question here: how to create three partitions, how to install different versions of macOS on different partitions (the answer differs depending on macOS versions)
    – nohillside
    Jun 6, 2021 at 13:22
  • I wouldn't recommend sharing home folders across different macOS versions. Anything in ~/Applications that is updated runs the risk of not being compatible with the older version of macOS when you switch back.
    – Scottmeup
    Jun 6, 2021 at 14:35
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    Yes, that's a good way to get your Photos library destroyed! BTDT! Jun 6, 2021 at 17:32

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The problem is that different OS versions may define or parse 'user configuration data' in different ways. So keys in .plist files might use different names; the range of acceptable parameters for a key might vary.

There are also other data types, such as sqlite3 databases, which might similarly have different tables, different names, and different data.

For example, 'downgrading' the data for Mail.app to an older version may not be possible, or lead to data corruption.

However, simply having a common area for documents is common practice -- as long as your OSes don't have different versions of applications that might use different data formats.

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