Two volumes in Disk Utility? (macOS Catalina 10.15) [duplicate]

Solution 1:

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.

Solution 2:

Here is the layout for Catalina storage from

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

• https://eclecticlight.co/2019/10/08/macos-catalina-boot-volume-layout/

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

• https://developer.apple.com/videos/play/wwdc2019/710/
• https://eclecticlight.co/2019/06/22/why-catalina-has-got-a-read-only-system-volume/

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