With macOS Catalina, Apple changed the structure of macOS on a boot disk drive. Instead of a single mixed set of system and data files, where Apple tried to ensure the integrity and immutability of the system files, the company divided it into two parts: a system volume and a data volume.
The two are merged into one “volume group”, an organizational method added in Catalina. When you start macOS Catalina or later, the Finder boot disk appears as a single device; behind the scenes are the two: “Volume Name” and “Volume Name – Data”.
If you are trying to delete your boot volume completely – for example to sell it to someone or as a prey – you can make use of an easily overlooked option in the Disk Utility labeled Erase Volume Group.
Follow these steps to delete your boot volume group:
Launch your Mac in recoveryOS. On an Intel Mac, restart or boot while holding down Command-R; with an M1, turn off the Mac, and then hold down the power button until “Loading startup options” appears.
Navigate to macOS Recovery, which may involve selecting a user account and entering its password to unlock boot volume.
click Disk Utility at the bottom of the macOS Recovery screen.
Select one of the two volumes in the container for the system (see figure).
Note that while the main volume is filled in and its name can be changed and there is a prominent set of Cancel / Delete buttons, look in the lower left corner for Delete volume group button. Click on it. (This option does not appear if you have selected the container or drive on which the volume group is displayed.)
Confirm the action.
Now both your system and data volumes have been deleted, giving a fresh start for a new owner or new macOS installation.
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