Perform a Clean Install of OS X Yosemite on Your Mac

OS X Yosemite Presentation
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Perform a Clean Install of OS X Yosemite on Your Mac's Startup Drive

OS X Yosemite on MacBook Pro
Freshen up your Mac with a clean install of OS X Yosemite. Courtesy of Apple

When you're ready to install OS X Yosemite you will find the version of Yosemite available for downloading from the Mac App Store supports two primary methods of installation: a clean install, which we'll show you how to perform in this guide, and the more common upgrade install, which we cover in detail in our step-by-step guide:

How to Upgrade Install OS X Yosemite on Your Mac

The clean method of installing OS X Yosemite wipes all of the data from the destination drive and replaces it with the fresh, never-before-used data from the OS X Yosemite installer. Gone are all your user data and any applications you installed.

While the clean install option may not sound like a very friendly way to update your Mac to OS X Yosemite, it does offer some advantages that can make it the preferred update path for some Mac users.

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, then a clean install may be a good choice.

Many of these perplexing problems can occur over the years of using your Mac. As you upgrade systems and applications, debris gets left behind, files become overly large, causing slowdowns, and some files used by the system or apps can become corrupt, slowing things down or even preventing your Mac from operating correctly. Finding these bits of file debris is almost impossible. If you're having these kinds of problems with your Mac, then a good clean sweep, as it were, may be just the remedy you need.

Of course, the cure may be worse than the problems. Performing a clean install will delete all of the data on the destination drive; if the destination is your startup drive, which for most of us it will be, then there go all of your personal data, settings, preferences, and apps. But if a clean install does indeed cure the problems, then the tradeoff may be worth it.

First, Back Up Your Data

No matter which installation method you choose, before you proceed, back up all of your data. A recent Time Machine backup is the bare minimum you should have on hand. You should also consider creating a clone of your startup drive. That way if anything terrible should happen, you can quickly recover by booting from the clone, and be right 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 some of your information over to your new installation of OS X Yosemite. Yosemite's 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

  • The Yosemite installer, available as a free download from the Mac App Store. If you're having problems finding 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 or later, and that you're willing to erase.

If you're wondering why we're mentioning OS X Snow Leopard, it's because Snow Leopard is the oldest version of OS X that supports the Mac App Store, which you must be able to access in order to download the Yosemite installer.

Let's Get Started

You did finish the backup, right? Okay; let's move on to the next page to start the installation process. 

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

Yosemite Install - OS X Utilities Window
You will use the Yosemite installer to erase yor Mac's startup drive. Screen shot courtesy of Coyote Moon, Inc.

With the preliminary steps out of the way (see Page 1), you're ready to download OS X Yosemite from the Mac App Store. Yosemite is a free upgrade for anyone running OS X Snow Leopard (10.6.x) or later. If you're using a version of OS X older than Snow Leopard and wish to upgrade to Yosemite, you must first purchase and install OS X Snow Leopard before you can upgrade to OS X Yosemite. If your using a newer version of the Mac OS and are contemplating downgrading to Yosemite consider the information in the article: Can I Upgrade or Downgrade to OS X Snow Leopard (OS X 10.6)?

Although written for Snow Leopard the information contained in the downgrade section is relevant to anyone choosing to go back from a newer version of the Mac OS to an earlier one.

Download Yosemite From the Mac App Store

  1. Launch the Mac App Store by clicking its icon in the Dock, or by double-clicking the App Store application located at /Applications.
  2. To find OS X Yosemite, click the Apple Apps link under the All Categories section of the right-hand sidebar. You may also find OS X Yosemite featured at the top of the All Categories section, or in the Featured products banner section of the Mac App Store. If you're reinstalling Yosemite check out the guide: How to Re-Download Apps From the Mac App Store for the needed instructions.
  3. Once you locate the OS X Yosemite app, click its download button. You may be asked to sign in if you haven't already done so.
  4. The Yosemite app file is in excess of 5 GB in size, so you may want to find something else to do while you wait for it to finish downloading.
  5. Once the download is complete, the OS X Yosemite Install app will launch 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 be erasing your startup drive as part of the clean install process. In order to erase and reformat the startup drive, you'll need to start up your Mac from another device. Since all OS X installers include Disk Utility and an assortment of other apps, booting from the Yosemite installer serves to not only allow you to erase the startup drive, but also perform the actual installation, all from the same USB flash drive.

You'll find detailed instructions on the process in the article:

How to Make a Bootable Flash Installer of OS X or macOS

Once you've finished 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

  1. Make sure the USB flash drive you created in the step above is still plugged directly into your Mac. Don't use a USB hub or plug the flash drive into your keyboard or display's extra USB ports; instead, plug the flash drive directly into one of the USB ports on your Mac, even if it means disconnecting some other USB device (other than your keyboard and mouse).
  2. Restart your Mac while holding down the option key.
  3. The OS X Startup Manager will appear on the display, showing all of the devices that you can boot your Mac from. Use the arrow keys to highlight the USB Flash Drive option, and then press the enter key to start your Mac from the USB flash drive and the OS X Yosemite installer.
  4. After a short time, you'll see the Yosemite installer's Welcome screen.
  5. Select the language you wish to use for installation, and then click the Continue button.
  6. The OS X Utilities window will display, with options for restoring a Time Machine Backup, Installing OS X, Getting Help Online, and using Disk Utility.
  7. Select Disk Utility, and click the Continue button.
  8. Disk Utility will open, with your Mac's drives listed in the left-hand pane. Select your Mac's startup drive, usually named Macintosh HD, and then click the Erase tab in the right-hand pane.
  9. WARNING: 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.
  10. Use the Format drop-down menu to ensure that Mac OS Extended (Journaled) is selected, and then click the Erase button.
  11. You'll be asked if you really want to erase the Macintosh HD partition. Click the Erase button.
  12. The startup drive will be completely erased. Once the process is complete, select Quit Disk Utility from the Disk Utility menu.
  13. You'll be returned to the OS X Utilities window.

You're now ready to start the actual OS X Yosemite installation process. Proceed to the next page. 

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Clean Install of OS X Yosemite: Complete the Installation Process

OS X Yosemite Welcome
The Yosemite Installer supports multiple languages and locations. Chose your location from the list. Screen shot courtesy of Coyote Moon, Inc.

In the previous steps, you erased your 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 all of the OS X Yosemite system files to your selected startup drive. Once everything has been copied, your Mac will reboot into Yosemite, and walk you through the final leg of your journey: setting up your admin account, migrating data from a previous version of OS X, and other general housekeeping tasks.

Start the OS X Yosemite Installation

  1. In the OS X Utilities window, select Install OS X, and click the Continue button.
  2. The OS X Utilities window will be dismissed, and the Install OS X app will be launched. Click the Continue button.
  3. The Yosemite software licensing terms will display. Read through the licensing terms, and click the Agree button.
  4. A panel will display, asking you to affirm that you really have read and agree to the terms. Click the Agree button.
  5. The installer will display the drives you can install OS X Yosemite on. Highlight the drive you wish to be your OS X Yosemite startup drive, and click the Install button.
  6. The installer will prepare your Mac for the installation of OS X Yosemite by copying files to your startup drive. Once the copying process is complete, your Mac will restart. An ongoing estimate of the time remaining until the restart will display during the file copy process. I've never known these time estimates to be accurate, so be prepared to wait longer than expected. You can go do something else if you wish. The first phase of the installation process, including the upcoming restart, will continue without any input required from you. It's not until after the restart that you'll be asked to help set up your Mac's basic configuration, and your Mac will be happy to wait patiently for you to return.
  7. Once the restart occurs, your Mac will display a new status message indicating the time it will take to complete the installation process on the startup drive. Once again, be prepared to wait.
  8. With all the files finally copied, a second restart will occur. Your Mac will boot to OS X Yosemite, start the setup assistant, and display a welcome screen.
  9. Select the country for the installation, and click Continue.
  10. Select the keyboard layout to use, and click Continue.
  11. The Migration Assistant will display, allowing you to transfer personal data from a Mac, Time Machine backup, another startup disk, or a Windows PC. At this time, I suggest selecting the "Don’t transfer any information now" option. You can always use the Migration Assistant later if you want to move data over to your new installation of OS X Yosemite. Remember, one of the reasons for a clean install is to not have older files present that may have caused problems in the past. Click Continue.
  12. Sign in with your Apple ID. This optional sign-in will preconfigure your 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 real time saver. However, you can also skip this step and sign in to these services later. We're going to assume that you wish to sign in with your Apple ID. Fill in the requested information, and click Continue.
  13. You'll be asked if it's okay to enable Find My Mac, a service that uses location information to help you find a lost Mac, or to erase the contents of your Mac if it's stolen. Make your selection.
  14. Additional licensing terms for various apps, such as iCloud, Apple's privacy policy, and the OS X software license will display. If you agree to the terms, click the Agree button.
  15. You'll be asked if you really agree; click the Agree button.
  16. It's now time to create your administrator account. Enter your full name and an account name. The account name will become the name of your home folder, and is also called the shortname for the account. I suggest using an account name with no spaces, no special characters, and no upper case 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 click Continue.
  17. OS X Yosemite makes use of iCloud Keychain, a system of storing encrypted keychain date between multiple Macs on which you have accounts. The process of setting up the iCloud Keychain system is a bit involved. I recommend using our guide to setting up and using iCloud Keychain at a later time; after all, you want to start using OS X Yosemite as soon as possible. Select Set Up Later, and click Continue.
  18. You'll be asked if you wish to make use of iCloud Drive. Do not set up iCloud Drive if you need to share iCloud data with a Mac running an older version of OS X, or iOS devices running iOS 7 or earlier. The new version of iCloud Drive is not compatible with older versions. WARNING: If you turn on iCloud Drive, all data stored in the cloud will be converted to the new data format, preventing older OS X and iOS versions from being able to use the data. Make your selection, and click Continue.

Your Mac will finish the setup process and then display your new OS X Yosemite desktop. Have fun, and take time to explore all the new features.