"This Disk is Locked" Reinstalling macOS

[Skeptical.me]

"This Disk is Locked" Reinstalling macOS​

I have a 2017 iMac 27" 5K with a 1TB Fusion Drive.

I have always used an external SSD for the boot drive.

Now I am selling it, I am trying to install macOS on the Fusion Drive.

When I boot into Recovery and try to install macOS I'm asked what drive I want to install macOS on. But the "Mac HD" has the message "The Disk is Locked".

So then I exit out of this and select "Disk Utility", I select "Mac HD" then "Erase" but the only options for format are "APFS" and not "Mac OS Extended".

And if I select "APFS" and try to reformat the drive nothing happens.

I really have no idea what to do at this point.

See images.

 

[Skeptical.me]

What I did to resolve this:

Booted into macOS using external SSD Drive.

Opened Disk Utility.

Selected (Fusion Drive) Mac HD in the side bar.

Selected "Edit"

Then selected "Delete APFS Volume..."

Then once that was completed I selected "Erase".

Then selected "Mac OS Extended (Journaled)" to reformat the drive.

Then I proceeded to Download the macOS High Sierra Installer.

I then created a macOS High Sierra Bootable Drive on a USB Drive.

Finally I pressed "Option" key at boot and booted up using the installer and proceeded with installation via Recovery.


I followed the instructions found here (see link) to create the bootable USB Drive.

Create a bootable installer for macOS - Apple Support

Use an external drive or secondary volume as a startup disk from which to install macOS.

support.apple.com