I’ll introduce how to completely erase a macOS disk.
macOS Clean Install Method (Standard Version)
Mac's "Disk Utility" equipped with SSD drives cannot use the "Secure Erase Options" because performing a standard disk "erase" makes it difficult to recover data from SSDs. 「SSDドライブを搭載したMacの「ディスクユーティリティ」では「確実な消去オプション」を利用することはできません。標準的なディスクの「消去」を実行すればSSDからデータを復元することは困難になるためです。」
As written, this is usually OK by itself.
macOS Clean Install Method (Complete Deletion Version)
Preparation
Prepare an external HDD/SSD or USB memory.
After downloading the installer, connect the USB flash drive or other volume you plan to use as the bootable installer. Prepare one with at least 12 GB of free space and formatted with Mac OS Extended. 「インストーラをダウンロードしたら、起動可能なインストーラとして使う予定の USB フラッシュドライブやその他のボリュームを接続します。空き容量が 12 GB 以上あり、Mac OS 拡張でフォーマットされているものを用意してください。」
In my case, I initialized an external HDD that was sleeping at home with “Mac OS Extended (Journaled).”
Creating macOS Installer
Specify /Volumes/UNTITLED where the external HDD is mounted as the location to create the installer.
$ sudo /Applications/Install\\ macOS\\ Mojave.app/Contents/Resources/createinstallmedia --volume /Volumes/UNTITLED
Password:
Ready to start.
To continue we need to erase the volume at /Volumes/UNTITLED.
If you wish to continue type (Y) then press return: Y
Erasing disk: 0%... 10%... 20%... 30%... 100%
Copying to disk: 0%... 10%... 20%... 30%... 40%... 50%... 60%... 70%... 80%... 90%... 100%
Making disk bootable...
Copying boot files...
Install media now available at "/Volumes/Install macOS Mojave"
Starting macOS in Recovery Mode
First, after shutting down, start macOS while pressing [option + power button].
Select the [Install macOS Mojave] disk.
macOS Disk Complete Erase Command
diskutil secureErase
This time, to execute “35 overwrites with Gutmann algorithm,” I specified 3 and executed it.
diskutil secureErase 3 /dev/disk1s1
Side Note: How Long Does '7 Overwrites' Take?
For how long the “7 overwrites” conforming to US Department of Defense (DoD) 5220-22-M standard takes, Time Required for Secure Erase with Security Options in Disk Utility has detailed verification methods by HDD/SSD capacity, so refer to this.
That’s all about wanting to completely erase macOS disk from the Gemba.