As on operating systems Windows, formatting the entire hard disk, either classic HDD or SSD, can not be made with the operating system turned on. It's normal. It would not even be possible to delete the system files while it was running.
Full formatting of the disk on Mac is done using utility programs from Recovery Mode. These tools are available regardless of the status of the hard drive. For example, when you change your HDD or SSD to a Mac, Recovery Mode entry is required to create the partition and install the operating system. It is equally necessary to start a Mac in Recovery Mode and when we want to completely delete the disk space and reinstall the operating system.
How to open a Mac in Recovery Mode / MacOS Recovery
There are three ways we can start a Mac in Recovery Mode, or macOS Recovery, as is known. Each variant will open the same utilities, but it will be different operating system to be installed.
Shut down all the Macthen press the power button. Immediate press and hold the key combinations below, depending on the macOS version you want to install.
Command (⌘)-R : You can reinstall the latest MacOS version on Mac.
Option-⌘-R : Do it upgrade to the latest version of macOS compatible with your Mac
Shift-Option-⌘-R : Install macOS version with which the device came from the factory. If that version is no longer available, install the closest version.
How to format / delete all data on a hard disk on a Mac
Of course, before this process you must Have a backup copy of all your data. These will be eradicated.
from macOS Utilities, we go and open disk Utility.
In disk Utility, click on "View"→"Show All Devices". The option is in the upper left corner.
We select the physical hard disk that we are going to format - it is in the tree above all partitions or volumes. Click "Erase"At the top of the Disk Utility, then give it to yourself a name main partition, choose formatting si hard schemeacquis.
A box of options opens. For the first layout, it's best to choose formatting: "Mac OS Extended (Journaled)". For SSDs, the first container partition will automatically become APFS volume.
Click Erase and wait for the operation to finish. Click "Done" at the end.
After this operation, the hard drive will be empty and you will need to reinstall the operating system. Close Disk Utility and go to "Reinstall macOS"How to format / delete all data on a disk (HDD or SSD) on macOS