Mac Re-install the OS

External

• https://support.apple.com/en-us/HT204904

Internal

• Mac

Re-install with Data Deletion

El Capitan: Command + R while restarting.

Other: Command + Option + R while restarting.

OS X Utilities -> Disk Utility

Reinstalling without explicit deletion of the partition will upgrade and preserve the data.

First delete the partition from Disk Utilities: macOS Utilities -> Disk Utility -> Select Internal -> <Partition Name> from the left tab. Usually the partition is named "Macintosh HD" -> Erase -> Specify the new name and the format: APFS (Case-sensitive). It is OK to leave the name as it is "Macintosh HD", this will be also the name of the new partition -> Erase. The process should end with a message similar to "Erasing "<Old Partition Name> and creating "<New Partition Name>" -> Done.

Then go back to macOS Utilities and install.

Adding a new volume to the container

Apple File System (APFS) is preferable, as it's the newest and optimized for SSDs.

Select the container disk or "APPLE SSD ... MEDIA".

Add a new volume with the + icon.

Name

APFS Case Sensitive

Installation

Then go back to OS X Utilities -> Reinstall OS X.

The computer must be connected to the Internet (wire or wireless): Utilities -> Network Utility.

If I get "The recovery server could not be contacted", it is probably because the time is out of sync, so go to Utilities -> Terminal and

ntpdate -u time.apple.com

"Reinstall OS X".

Select the drive -> Install.

Create a Bootable USB

https://support.apple.com/en-us/HT201372