You need to delete the volume group or Macintosh HD - Data when factory resetting a Mac.
Macs formatted with APFS (the majority of Macs) have a volume group consisting of Macintosh HD (stores the operating system and built-in apps) and Macintosh HD - Data (keeps user data). The distinction between the two volumes is made to reduce human interference with system files and enable users to reinstall macOS without data loss.
If you only erase the Macintosh HD volume, the system files are deleted but the real storage killer - files on the Macintosh HD - Data volume are still intact. That's why you need to delete the volume group when reinstalling macOS. Follow the steps below to format your startup disk and reinstall macOS:
- Shut down your Intel Mac.
- Hit the power button, then immediately press and hold the Command + R keys until seeing the Apple logo.
- Click Disk Utility and Continue.
- Select Macintosh HD and hit the ( - ) button.
- Click "Delete Volume Group."
- Click View > Show All Devices.
- Select the drive (Apple SSD) and click Erase.
- Name it Macintosh HD.
- Keep the default format and scheme.
- Click Erase.
- Close Disk Utility.
- Click "Reinstall macOS" and Continue to install a new operating system.
