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
- Navigate to macOS Recovery, which may involve selecting a user
account and entering its password to unlock the startup
- Click Disk Utility at the bottom of macOS Recovery's
- 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
- 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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