Mac Re-install the OS

External

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

Internal

• Mac

Re-install with Data Deletion

Control + R while restarting -> OS X Utilities

First delete the partition from Disk Utilities, then "Reinstall OS X". Reinstalling without explicit deletion will upgrade and preserve the data.

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

Command + R while restarting.

Create a Bootable USB

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