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I created a second bootable volume called "Beta" in order to install the macOS Catalina beta. I successfully installed and booted to this new volume, but when I returned to the first volume (which has macOS Mojave installed) I was surprised to see a third volume.

About This Mac showing third volume

The "Beta - Data" seems to have been created during the installation process. This third volume is also not visible when I return to the "Beta" boot volume, only having the "Beta" and "Panther" volumes. The "Beta" volume when booted into it also seems to have the combined size of the "Beta" and "Beta - Data" volumes.

I originally created the "Beta" volume with a quota of 256GB as well, but that quota is gone from that volume and the "Beta - Data" volume seems to have inherited it.

The "Beta" volume seems to contain the system install; containing the home, Library, System and Users directories (though the Users directory only contains the Shared directory, and not the home folder for the user I created). While the "Beta - Data" volume has the user-specific data; having the Device, private and Users directories (with the home folder for the user I created within, as well as another Shared directory).

I have never tried dual booting on a Mac before, so this could be more common than I am aware of, but I couldn't seem to find any information on why this would happen or where this volume came from. Could this be a new "feature" (or bug) from macOS Catalina? Or did I just do something wrong when I created the new volume?

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macOS Catalina has a new feature to further isolate the OS from data. In the APFS container it is installed within, an APFS volume for the OS is created alongside one for your data. The former is mounted read-only within Catalina, and the latter has the Data postfix containing your apps and data.

https://developer.apple.com/videos/play/wwdc2019/103/

Within Catalina, Disk Utility shows these volumes with a Finder icon and home icon to distinguish them. This does not appear in Mojave, showing as normal volumes. Keep in mind the release notes for the beta of Catalina, specifically dual booting and Spotlight indexing, which will heavily confuse Mojave with the cross-volume linking of various folders.

  • Do you have references for the Mojave confusions? I couldn't find anything related to dual booting, spotlight indexing or cross-volume linking in the release notes of Catalina beta 1 – flo_23 Jun 7 at 8:14
  • @flo_23 I don’t want to quote the release notes here since they’re not public yet, but look for the heading for Mail and its known issues in the Catalina beta 1 release notes, it’s just over half way down (cmd-f ‘mail’ and 3 of the 4 matches are regarding this issue). It’s not just Mail — many 3rd party apps are confused too. – grg Jun 7 at 8:20
  • Found it, thanks! So creating a partition instead of an APFS volume in the same APFS container as Mojave should resolve these issues, correct? – flo_23 Jun 7 at 11:55
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Here is the layout for Catalina storage from

Eclectic Light diagram of macOS Catalina System Volume Layout (Macintosh HD)

Reading the entire article for a very excellent explanation and mapping of firm links, and much more details is well worth it for anyone interested in storage on macOS

The system volume is read only. Your data and areas that are write capable are on the Data volume. This is by design.

There are three non-visible containers for each Catalina capable APFS container and two visible volumes per runnable OS. In my case below I have a Catalina install "Macintosh HD - I call it slim" with all my data and a clean install of Catalina "Catalina" so the three + two + two = seven containers in the "synthesized" disk1 - all sharing space in the main physical APFS Container disk0

/dev/disk1 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +250.0 GB   disk1
                                 Physical Store disk0s2
   1:                APFS Volume slim - Data             166.5 GB   disk1s1
   2:                APFS Volume Preboot                 184.3 MB   disk1s2
   3:                APFS Volume Recovery                1.6 GB     disk1s3
   4:                APFS Volume VM                      3.2 GB     disk1s4
   5:                APFS Volume Catalina - Data         3.7 GB     disk1s5
   6:                APFS Volume Catalina                10.9 GB    disk1s6
   7:                APFS Volume slim                    10.7 GB    disk1s7
  • so "Macintosh HD" is the system volume & "Macintosh HD - Data" is my data on the harddrive(ie folders & files)? – no nope Oct 9 at 4:34
  • How to access Macintosh HD - Data like used to to Macintosh HD? – Volatil3 Oct 9 at 7:58
  • @Volatil3 That probably needs to be fleshed out as a follow on question or something to work out what you're asking in Ask Different Chat – bmike Oct 9 at 11:23

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