Performing a clean install will erase the Mac and you will lose any data you do not save to an external volume Be sure to back up your important files, pictures, data, or whatever else – do not skip this. Alternatively, perform your own backups manually. Start and complete a backup with Time Machine before beginning.How to Clean Install OS X El Capitan & Start Fresh If you don’t know what you’re doing with a clean install, and you have not made backups, don’t try this, you will lose data. This is generally best for advanced users. If you are choosing to do this, you need to backup your data yourself, as erasing the volume and performing a clean install means nothing is coming with it. Remember, a clean install will erase everything, format the hard drive, and then install OS X El Capitan with nothing else on the Mac. If you have already installed OS X 10.11 on the Mac, you can download it again and create a boot installer with these instructions.

You will need to have made a bootable OS X El Capitan installer drive with a USB flash key to perform a clean install. While most Mac users are well served by running the normal update process to OS X El Capitan after it has been downloaded from the Mac App Store, this guide is for users who want to start fresh.

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