How to Perform a Clean Install of OS X El Capitan
OS X El Capitan (OS X 10.11) offers two installation methods. This guide focuses on the “clean install” method. When you install El Capitan on your current startup drive with the clean install method, you erase everything on the drive. That includes OS X, your user data, and personal files. Back up your data before you begin. After you back up your data, download El Capitan from the Mac App Store. Copy the installer to a USB drive to make a bootable drive if you’re installing to the startup drive. If you’re performing the clean install on an empty volume, you can jump to the section titled, “Perform a Clean Install of OS X El Capitan.” You don’t need a bootable USB drive.
Erase the Startup Volume
After you back up your data and create a bootable USB drive containing El Capitan, erase your Mac’s current startup drive by following the steps below.
Perform a Clean Install of OS X El Capitan
If you chose to perform a clean install on your current startup drive, then you have already erased your startup drive and started up the installer. If you chose to perform a clean install on a new or empty volume (not your startup drive), then you’re ready to start the installer, which you’ll find in the Applications folder. The file is labeled Install OS X El Capitan. The installation processes are the same going forward for both clean install methods.
Set Up OS X El Capitan
When the installation process is complete, your Mac reboots, and the El Capitan setup assistant automatically starts. The assistant helps you through the process of configuring your Mac and operating system. If it isn’t correct, select Show All Disks and select the correct target disk. Choose Install. Enter your administrator password and select OK.
Why Perform a Clean Install?
The clean install method is a good choice for testing a new OS on a dedicated drive or partition, or when you have been experiencing software related issues with your Mac that you have not been able to resolve. When the problems are severe enough, you may be willing to trade in your apps and data for the peace of mind of a clean slate.
Types of Clean Installs
There are two types of clean installs you can perform: an install onto an empty volume, and an install on a startup volume.
Clean Install on Empty Volume
This involves installing El Capitan onto an empty volume, or at least one whose contents you don’t mind removing. The key point is that you’re not targeting your current startup volume as the destination for the clean install. This type of clean installation is easy because, since the startup drive isn’t involved, you can perform the clean install while booted from the current startup drive. There’s no special, custom-made startup environment needed. Just start up the installer and go.
Clean Install on Startup Volume
The second option, and perhaps the more common of the two, is to perform a clean install on the current startup drive. Because the clean install process erases the contents of the destination drive, it’s obvious that you can’t boot from the startup drive and then try to erase it. The result, if it were possible, would be a crashed Mac. That’s why if you choose to clean install El Capitan on your startup drive, there’s an extra set of steps involved: creating a bootable USB flash drive that contains the El Capitan installer, erasing the startup drive, and then starting the clean install process.
Back Up Your Existing OS and User Data
By installing El Capitan on your current startup drive with the clean install method, you will erase everything on the drive. That includes OS X, your user data, and personal files. No matter the method, you should have a current backup of the existing startup drive’s contents. You can use Time Machine to perform this backup, or one of the many cloning apps, such as Carbon Copy Cloner, SuperDuper, or Mac Backup Guru. You can even use Disk Utility. Before proceeding with the installation, it’s important to take the time to create a current backup.
