Saturday, April 9, 2022

How to erase Mac SSD for downgrade macOS to older version

It used to be using created bootable USB stick in order to downgrade macOS to older version, as in here.

But with the new macOS Monterey and with the T2 Security Chip of modem Mac, your Macintosh SSD will be encrypted and for Intel Mac, the bootcamp partition will also encrypted.

As a result, you cannot install macOS from USB stick to the encrptyed SSD, even if you have enabled boot from media in the Startup Security Utility.

The downgrade path will now require you to erase all content and settings first.
If you haven't do so, you have to do Erase Mac as below before installing any macOS version.
In addition, if you want to downgrade macOS, you have to do

Step (1) Backup all your important data and be sure you have a good Internet connection for the remaining steps

Step (2) Erase Mac, reboot Mac and select Erase Mac from the menu. This will wipe and remove all the whole Macintosh SSD, including the encrypted Macintosh partition and bootcamp partition if any, you might need to authenticate by entering the admin password of your existing MacOS to do this. You can use Internet Recovery to the original macOS version (Option-Shift-Command-R), or else it will default to the latest macOS Monterey (Option-Command-R).

Step (3) Activate Mac, reboot Mac and enter Recovery by pressing Command-R and activate the Mac.

Step (4) Internet Recovery, reboot Mac and enter Recovery by pressing Command-R and use Internet Recovery to choose install original version of macOS.

Step (5) Enable boot from media, After successful installation of the old macOS version, create bootable USB stick of your required macOS version based on this guide from Apple. Reboot Mac and enable boot from media from in the Startup Security Utility.

Step (5) Install macOS from USB stick, reboot again and press Option to choose the USB stick to install from macOS.

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