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On an APFS volume, can I have two boot containers (beta 10.15.x on one and 10.14.x on the other) that share a common data container so that I can switch between startup disks while keeping the same home folder & data?

Or is there a better way to accomplish the same goal?

Sorry. I used "container" incorrectly. I should have said:

...can I have two boot volumes (beta 10.15.x on one and 10.14.x on the other) that share a common data volume so that I can switch between startup disks while keeping the same home folder & data volume?

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I would say you need 2 systems and one third volume to do this the right way. It’s long been possible and sometimes preferred to have your user home folder apart from the boot system.

  • 10.14 has a combined default user data and system - add your main user with a home folder outside that, but shared with APFS so that all the free space combines.
  • 10.15 has a read only system and a paired user data volume with firm links. Add the same main user outside these volumes.

You will certainly have some breakage and possibly much breakage when you use things like Photo Library, podcasts, music and more when the 10.15 system reads the data structure and modifies / migrates then in the user folder on disk to a newer structure. When the OS boots back to 10.14 this breakage will show and depending on your needs, you might need to keep two parallel home systems, so I would make a good backup of the 10.14 side before you boot to 10.15 and open anything more complicated than isolated documents.

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  • You could also set up one system & account normally, then edit the other's home folder to point to it (e.g. on 10.14, set the home folder to something like /Volumes/Catalina HD - Data/Users/username. Note that you also need to make sure the user ID numbers match between the two systems. Jan 28 '20 at 0:55
  • Please add that most Apple stuff will be broken: e.g. iTunes (10.14) vs. Music, Podcast etc. (10.15)
    – klanomath
    Jan 28 '20 at 6:30
  • @klanomath I’ve edited, feel free to help with grammar and details if you like. Thanks for the solid note.
    – bmike
    Jan 29 '20 at 23:15
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    I haven't tested all changes (data structures/internal file changes/sandbox changes etc.) thoroughly. Generally I would say: set up as a consumer Mac (mostly Apple apps) it's no good idea to use one shared user folder for both systems. Setting it up/using it as a pro Mac (science|graphics|coding|etc.) and avoiding Apple apps (I think even Mail's (10.15) "upgraded" user mail folder won't work with Mail (10.14)) is more promising.
    – klanomath
    Jan 29 '20 at 23:27
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It seems a volume group cannot be rebind after creation.

diskutil apfs addVolume disk1 apfs “Test-Data” -role D
diskuitl apfs addVolume disk1 apfs “Test” -role S -groupwith disk1s[X]
diskutil apfs listvolumegroups

Also Mojave might not understand volume group at all.

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  • IMO unless you do need Catalina features, stay at Mojave until 10.15.6 release.
    – amdyes
    Jan 30 '20 at 14:06
  • This appears to be the "opposite" of a question with an xy-problem: an answer with a yx-solution.
    – klanomath
    Jan 30 '20 at 17:40

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