IMPORTANT: before erasing the MacOS operating system, make sure to turn on Allow boot from external media.

The MacOS Recovery screen has functionality to:

  • Delete the existing OS on hard drive in Disk Utility.
  • View/Change startup security settings in Startup Security Utility

and reinstall MacOS using one of:

  • Option 1: Download and install MacOS (requires computer online).
  • Option 2: Install MacOS from bootable external media (requires permission to boot from external media).

Option 2 is preferrable as this can be the latest MacOS version, which should be downloaded and written to bootable drive before the OS is erased. Option 1 will install the MacOS version that the computer was shipped with and requires internet connection.

Procedure

Reboot into MacOS Recovery screen and choose erase your MacOS hard disk drive.

Reboot > Wait for blank screen > Press CMD+R

Open Disk Utility > select Mackintosh HD volume > click Erase

After erasing the MacOS, reboot back to Recovery screen for next steps.