Mac Re-install the OS

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External

Internal

Re-install with Data Deletion

El Capitan: Command + R while restarting.

OS X Utilities -> Disk Utility

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

Local the drive you want to erase (usually it's the only one, and it's named "Macintosh HD".

Erase "Macintosh HD"?

You can preserve the current name, and choose a format: OS X Extended (Case-sensitive, Journaled) -> Erase.

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

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

Re-install without Data Deletion

Create a Bootable USB

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