With macOS Catalina, Apple changed macOS’s structure on a startup volume. Instead of a single commingled set of system and data files, with Apple trying to ensure the integrity and immutability of system files, the company split it into two parts: a system volume and a Data volume.
The two are melded together into a “volume group,” an organizational method added in Catalina. When you boot macOS Catalina or later, the startup volume appears in the Finder as a single entity; behind the scenes, they’re two: “Volume Name” and “Volume Name – Data”.
If you’re trying to erase your startup volume completely—say, to sell it to someone or as a trade-in—you can make use of an easily overlooked option in Disk Utility labeled Erase Volume Group.
Select a volume in the volume group within the container before clicking Erase (top) and the Erase Volume Group options appears as a choice (bottom).
To erase your startup volume group, follow these steps:
- Boot your Mac into recoveryOS. On an Intel Mac, restart or start up while holding Command-R; with an M1, shut down the Mac, 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 the startup volume.
- Click Disk Utility at the bottom of macOS Recovery’s screen.
- Select either of the two volumes in the container for the system (see figure).
- Click Erase.
- Note that while the main volume is filled in and its name can be changed, and there’s a prominent Cancel/Erase set of buttons, look to the lower-left corner for the Erase Volume Group button. Click that. (This option doesn’t appear if you have selected the container or drive on which the volume group appears.)
- Confirm the operation.
Now both your system and Data volumes are deleted, providing a fresh start for a new owner or new macOS installation.
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