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Reinstalling Mac OS X

If you've been instructed to reinstall system software on your Macintosh computer by your IT consultant or the IT Service Desk, here are the steps you'll need.

Before you begin, you will need the system software install media associated with the operating system you will be reinstalling. Typically, these are the CDs or DVDs that came with your computer, or were included in a system software upgrade purchase.

Is There Anything I Should Do or Know Prior to Reinstalling an Operating System?

You cannot reinstall an earlier version of Mac OS X over a later version unless you perform an archive install or reformat the computer.

  • Read the "Before You Install" or "Read Me" document associated with your operating system if one is available for the software you are installing. This may list the system requirements and any known problems.
  • If you receive a warning message about updating your computer's firmware, be sure to do so prior to installing the operating system. For the latest Firmware Updates, go to the Apple Downloads web page.
  • Allocate a couple of hours or more to reinstall Mac OS X, software updates, and your application software.
  • Back up all important documents to a separate device before beginning.
  • Have all application software disks or CDs readily available for reinstalling if you plan to do a reformat/reinstallation of Mac OS X. This involves deleting all files on your hard drive and copying an entirely new set of operating system files on to your computer. This replaces the previous operating system, and is preferred in general, as it will most likely eliminate previous system software problems.  In contrast, an archive install moves all older operating system files into a new folder called Previous System Folder.  It keeps your data intact but you will need to manually move preferences.

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Should I Check My Disk Drive for Problems before Reinstalling Mac OS X?

We strongly recommend you perform disk verification and repairs using the Disk Utility application prior to reinstalling Mac OS X. To verify and repair your hard drive, you will need to restart your computer from the Mac OS X System CD (disk 1) or DVD. To do this insert your Mac OS X software CD (disk 1) or DVD and restart your computer while holding down the [C] key.

  1. Once your computer has started up from the CD, on systems running Mac OS X up to 10.3.9, select Disk Utility from the Installer menu; on systems running Mac OS X 10.4 or higher, select Disk Utility from the Utilities menu.
  2. Click the First Aid tab.
  3. Select your Mac OS X volume(s). We highly recommend you repair disk permissions first, and then repair hard drive problems.
  4. Click Repair Disk Permissions to fix any file permission problems.
  5. Next, click Repair Disk to fix any hard drive problems.

If the Verify and Repair radio buttons are dimmed in the Disk Utility window, be sure you have selected a volume to check.

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What Should I Do If the Disk Utility Is Unable to Verify or Repair My Disk?

If Disk Utility cannot repair the problem(s) found, this may indicate a hardware problem. We strongly recommend you do not attempt to reinstall the operating system, but instead call the IT Service Desk (Help Desk) at 646-2999, send e-mail to, or contact your department's IT support office.

How Do I Reinstall the Operating System?

To reinstall Mac OS X (all versions) on your computer, insert your Mac OS X software CD (disk 1) or DVD  into the CD or DVD drive and restart your computer while holding down the [C] key. Once the computer starts up from the CD or DVD, the installation window appears. Click Continue in the first two windows that appear, then read the important information concerning the installation. Click Continue again in the next two windows, then click Agree if you accept the terms of the license.

Select the hard drive and volume you want to install Mac OS X on. When the volume has been selected, click on the Options button that appears on the bottom left-hand side of the installation window. You have the option of archiving or erasing the old operating system.

If you select Archive and Install, you can preserve your files and the users and network settings, as well as place the old operating system into a folder called Previous System, while installing a new copy of Mac OS X. Be sure to check the box labeled Preserve Users and Network Settings if you wish to preserve the files and settings from your previous system software installation.

If you select Erase and Install, all files, personal settings, folders, and software will be deleted when you erase the hard drive. If you choose to erase and reformat the disk at this stage, format it using the Macintosh extended file system.

Continue the installation, which will likely take a while. Insert additional CDs if they are requested. Once it finishes, you will be prompted to restart your computer. Upon restart, enter all the registration information and follow the prompts. When you are asked to create your account, enter a User Name and Password. Don't forget this password; you will need it if you ever want to perform additional installations and system updates. You may find it advantageous to include a hint should you forget the password.

Select the appropriate time zone, date, and time. The installation of the system software is now complete. When the computer starts up with the new operating system, you may be prompted to install software updates.  Run software update, then restart your computer. Check for additional software updates, install, then restart your computer.  Continue checking and installing until there are no software updates available.  After updating system software, any application software can now be installed.

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How Do I Apply Software Updates to My Operating System and What Should I Know about How This Process Works?

For Mac OS X 10.3 and higher, Software Update is available under the Apple menu.

You must have an Administrator user name and password to install software updates.

Make sure you have enough disk space. Even small application updates may require a few hundred megabytes of space to complete the optimization phase of the installation. Mac OS X itself requires up to 1.5 GB of free space.

You may want to avoid using your computer while the updates are being installed, although it is not required to install most updates. You may even want to restart your computer immediately before installing an update. If the Classic environment is running, turn it off; it will maximize the amount of available memory for the installation.

Some installations automatically restart the computer; you may not notice if you start the installation, then leave your computer.

Using the Software Update feature, you can get updates immediately or schedule when Mac OS X checks for updates. Because some updates are prerequisites for others, you may need to use Software Update more than once to get all the desired updates.

The basic installation steps are provided below. More detailed information is available; see Mac OS X: Updating Your Software.

Update Now

To immediately get updates for Mac OS X versions 10.3 and higher:

  1. Select Software Update from the Apple menu.
  2. In the Software Update window, select the items you want to install, then click Install.
  3. Enter an Administrator user name and password.
  4. Once the update has completed, restart the computer, if necessary.
  5. Repeat these steps to see if more updates are available. Because some software updates are prerequisites for others, you may need to repeat these steps several times to complete the software update sequence.
  6. Optional: You may want to make software you do not need "inactive." You will most likely encounter some updates that you do not need, such as support for languages that you do not use. When you choose not to install an update, you will still see it each time you use Software Update. If you want to hide them, see Mac OS X: How to Make a Software Update Inactive or Active (on Apple's Web site).

Schedule an Update

Software Update can be configured to open and look for updates immediately after you log on to your computer on a regular basis, assuming you are connected to the Internet. You can set this to occur daily, weekly, monthly, or not at all (not recommended).

To schedule updates for Mac OS X versions 10.3 and higher:

  1. Select System Preferences from the Apple menu.
  2. Select Software Update from the System area.
  3. Make sure the box next to Check for updates is selected.
  4. Select the interval you prefer (daily, weekly, or monthly) from the pull-down menu.
  5. You may also choose to Download important updates in the background by selecting this option.
  6. Close the System Preferences window.

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What Can I Do If the Automatic Software Updater Doesn't Appear to Be Working?

Sometimes you may need to download the standalone installer file for a software update. These are available from the Apple Downloads web page. The Software Update preference pane will force you to install software in the correct order, you should carefully read the stated system requirements before using a standalone installer. Be sure to install in the correct order and only on compatible systems when applicable. A standalone installer may be advantageous when you:

  • Anticipate needing the file again later.
  • Need to install on multiple computers and need a portable file.
  • Have a faster Internet connection available to you at another computer.
  • Have no Internet connection on the computer that needs the update.

Should I Check My Disk Drive for Problems after Installing Software Updates?

We recommend you perform permission and disk repair using the Disk Utility application after installing software updates. Follow the instructions above in Should I Check My Disk Drive for Problems before Reinstalling Mac OS X?

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Last Updated: 8/2/10