Perform a Clean Install of OS X Yosemite on Your Mac

The Benefits of Performing a Clean Install of OS X Yosemite

If your Mac suffers from annoying problems that you've been unable to fix, such as occasional freezes, unexpected shutdowns, applications that hang or seem exceptionally slow, or poor overall performance not attributed to hardware issues, a clean install may be a good choice.

Many of these problems can occur over the years of using your Mac. As you upgrade systems and applications, debris is left behind and files become large. This causes slowdowns and may corrupt some system files. Finding these bits of file debris is almost impossible. If you encounter these kinds of problems with your Mac, a good clean sweep may be the remedy your Mac needs.

Sometimes, the cure may be worse than the problems. Performing a clean install deletes all the data on the destination drive. If the destination is your startup drive, which for most users it will be, there goes personal data, settings, preferences, and apps. However, if a clean install cures the problems, the tradeoff may be worth it.

Back Up Your Data

No matter which installation method you choose, back up your data before proceeding. A recent Time Machine backup is the bare minimum you should have on hand.

Also, consider creating a clone of your startup drive. That way, if anything terrible happens, you can recover by booting from the clone and be back where you started, without taking the time to restore the data from a backup.

A clone is also an advantage when it's time to migrate your information to your new installation of OS X Yosemite. The Yosemite Migration Assistant works with cloned drives and lets you easily move data you may need.

What You Need for a Clean Install of OS X Yosemite

Here's what you'll need to perform a clean install:

The Yosemite installer on a disc or as a downloaded image file. If you can't find Yosemite in the store, check the Purchase page. If you ever downloaded Yosemite in the past, it will be listed there.
A Mac that meets the minimum requirements for OS X Yosemite.
A recent backup of your current Mac startup drive.
A startup drive that contains OS X Snow Leopard (10.6) or later and which you're willing to erase.

OS X Yosemite on MacBook Pro — Apple, Inc.

Clean Install of OS X Yosemite: Boot From USB Flash Drive to Start the Process

With the preliminary steps out of the way, you're ready to start the process.

If you use a version of OS X that's older than Snow Leopard (10.6) and want to upgrade to Yosemite, you must purchase and install OS X Snow Leopard before upgrading to OS X Yosemite.

Launch the Mac App Store by clicking its icon in the Dock or double-click the App Store application located under /Applications in Finder.
Navigate to the OS X Yosemite download page by searching the App Store for Yosemite.
Once you locate OS X Yosemite, select the Download button. You may be asked to sign in if you haven't done so.
When the download is complete, the OS X Yosemite Install app launches on its own. Do not proceed with the installation. Instead, quit the installer by selecting Quit Install OS X from the Install OS X menu.

Create a Bootable Version of the Yosemite Installer

Now that you have the OS X Yosemite installer downloaded to your Mac, the next step is to make a bootable copy of the installer on a USB flash drive. You need a bootable version of the installer because you'll erase your startup drive as part of the clean install process.

To erase and reformat the startup drive, start your Mac from another device. Since all OS X installers include Disk Utility and an assortment of other apps, booting from the Yosemite installer allows you to erase the startup drive and perform the installation, all from the same USB flash drive.

Once you finish creating a bootable version of the OS X Yosemite installer, come back here to continue the clean install of OS X Yosemite.

Boot From the USB Flash Drive

Follow these steps to boot the installer from a USB flash drive.

Make sure the USB flash drive you created in the step above is still plugged into the Mac. Don't use a USB hub or plug the flash drive into the keyboard or display's extra USB ports. Instead, plug the flash drive into one of the USB ports on the Mac.
Restart the Mac while holding down the Option key.
The OS X Startup Manager appears on the display, showing the devices you can boot the Mac from. Use the arrow keys to highlight the USB Flash Drive option, then press the Enter key to start the Mac from the USB flash drive and the OS X Yosemite installer. After a short time, you'll see the Yosemite installer's Welcome screen.
Select the language you want to use for the installation and then select Continue. The OS X Utilities window displays with options for restoring a Time Machine Backup, Installing OS X, Getting Help Online, and using Disk Utility.
Select Disk Utility and then select Continue. Disk Utility opens, with the Mac's drives listed in the left pane.
Select the Mac's startup drive, usually named Macintosh HD, and select the Erase tab in the right pane.

You are about to erase your Mac's startup drive and all of its contents. Make sure you have a current backup of this data before proceeding.
Use the Format drop-down menu to ensure that Mac OS Extended (Journaled) is selected and then select Erase.
You're asked if you want to erase the Macintosh HD partition. Select Erase.
The startup drive is completely erased. Once the process is complete, select Quit Disk Utility from the Disk Utility menu. You're returned to the OS X Utilities window.

You're now ready to start the OS X Yosemite installation process.

Clean Install of OS X Yosemite: Complete the Installation Process

In the previous steps, you erased the Mac's startup drive and returned to the OS X Utilities window. You're now ready to complete the installation process by letting the installer copy the OS X Yosemite system files to your selected startup drive.

Once everything is copied, your Mac reboots into Yosemite and walks you through setting up your admin account, migrating data from a previous version of OS X, and other general housekeeping tasks.

In the OS X Utilities window, select Install OS X and then select Continue.
The OS X Utilities window is dismissed, and the Install OS X app launches. Select Continue.
The Yosemite software licensing terms display. Read through the licensing terms and select Agree.
A panel displays, asking you to affirm that you read and agree to the terms. Select Agree once again.
The installer displays the drives you can install OS X Yosemite on. Highlight the drive you want to be the OS X Yosemite startup drive and then select Install.
The installer prepares the Mac for the installation of OS X Yosemite by copying files to the startup drive. Once the copying process is complete, the Mac restarts. An ongoing estimate of the time remaining until the restart displays during the file copy process. The first phase of the installation process, including the restart, continues without any input required from you. It's not until after the restart that you are asked to help set up the Mac's basic configuration.
Once the restart occurs, the Mac displays a new status message indicating the time it will take to complete the installation process on the startup drive. Be prepared to wait.
With all the files copied, a second restart occurs. The Mac boots to OS X Yosemite, starts the setup assistant, and displays a welcome screen.
Select the country for the installation and then select Continue.
Select the keyboard layout to use and then select Continue.
The Migration Assistant displays, allowing you to transfer personal data from a Mac, Time Machine backup, another startup disk, or a Windows PC. At this time, we suggest selecting the Don't transfer any information now option. You can always use the Migration Assistant later if you want to move data to your new installation of OS X Yosemite. One reason for a clean install is to not have older files present that may have caused problems in the past. Select Continue.
Sign in with your Apple ID. This optional sign-in preconfigures the Mac to use iCloud, iTunes, the Mac App Store, FaceTime, and other Apple-provided services. If you intend to use any of these services, signing in now is a timesaver. However, you can skip this step and sign in to those services later. We're going to assume that you want to sign in with your Apple ID. Fill in the requested information and select Continue.
You're asked if it's OK to enable Find My Mac, a service that uses location information to find a lost Mac or to erase the contents of your Mac if it's stolen. Make your selection.
Additional licensing terms for various apps, such as iCloud, Apple's privacy policy, and the OS X software license display. Select Agree to continue, then confirm the agreement by selecting Agree again.
It's now time to create your administrator account. Enter your full name and an account name. The account name becomes the name of your home folder and is also called the short name for the account. We suggest using an account name with no spaces, no special characters, and no uppercase letters. If you wish, you can also choose to use your iCloud account as your sign-in method. If you check the Use my iCloud account to log in option, you will log in to your Mac using the same details as your iCloud account. Make your selection and then select Continue.
OS X Yosemite makes use of iCloud Keychain, a system of storing encrypted keychain data between multiple Macs on which you have accounts. The process of setting up the iCloud Keychain system is a bit involved. We recommend using our guide to setting up and using iCloud Keychain at a later time. Select Set Up Later and select Continue.
You're asked if you wish to make use of iCloud Drive. Don't set up iCloud Drive if you need to share iCloud data with a Mac running an older version of OS X or iOS devices with iOS 7 or earlier. The new version of iCloud Drive isn't compatible with older versions. Make your selection and select Continue.

If you turn on iCloud Drive, all data stored in the cloud is converted to the new data format, preventing older OS X and iOS versions from using the data.