Just as some people climb mountains “because they’re there”, others do things to macOS because they’re possible. Ever since Catalina divided the startup volume into two in a special Volume Group, we’ve been using Disk Utility and its command line equivalent diskutil to delete one of those two grouped volumes but not the other. This article explains why that isn’t wise in Big Sur or Monterey.
To know what to expect when you delete one volume in a Volume Group we need to understand what makes that group.
Each of the two volumes within a Volume Group is designated as being a group member with its specific role and UUID. They’re also glued together by the firmlinks which make the two volumes work with one another. Not only that, but macOS has many essential files which are stored not on the System volume, but on the Data volume instead, where they can be written to. These most notably include Safari and several of the writable hidden top-level directories. Those are firmlinked into place and the whole Data volume mounted inside the System volume.
Read more at eclecticlight.co