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. If that leads to a wireless network setup and a "spinning globe" that doesn't end, try Command ⌘ + R.

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".

The partition may not be mounted, and it should be mounted with the "Mount" Disk Utility option.

Erase → Specify the new name and the format: APFS (Case-sensitive, Encrypted). 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