MAC - Format and set up a clean install of OS X

- Save your data on your extern disk

- Download the last OS X version through the App Store

- Format a USB key through the Terminal by command:

<form> sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/Sans\ titre --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app --nointeraction </form>

- Restart your computer and press the ALT button

- Optional: you may need to format your disk if you set up a new SSD hard disk for example (follow the steps though the Utilities Tools)

- Install your clean OS X