Mac Re-install the OS
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External
Internal
Re-install with Data Deletion
El Capitan: Command + R while restarting.
Other: Command + option + 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.