Both PowerPC- and Intel-based Macs ship with a certain version of Mac OS X (or Mac OS X Server) on their installation disc(s). You should not install a version of Mac OS X earlier than that which came with your Mac.
If you install an earlier (previous) version of the Mac OS X than what was included, your computer may exhibit unexpected behavior such as:
- The trackpad or mouse may not respond properly
- The computer may stop responding
- Sleep/wake issues may occur
- The display image may appear to "shrink" with black bars around it, may appear tinted, or have other issues
- Loss of built-in audio
- Loss of Bluetooth or AirPort functionality
- May not start up past the Apple logo
See this article to learn if the Mac OS X or Mac OS X Server version on your Mac is what originally came with the computer.
If you are installing an earlier Mac OS X version from a disc
If an alert message appears stating you are using an incorrect disc, then you should not try to use that disc with your Mac.
If you already installed a previous version of Mac OS X and are having issues with your computer, you should reinstall Mac OS X with the installation disc(s) that came with your Mac (or use installation discs containing a later, compatible version of Mac OS X or Mac OS X Server). Consult the user guide that came with your computer for more information.
If you are trying to restore a Time Machine backup that was made on a different Mac
If you receive an alert message that your Time Machine backup is from an older Mac OS X version (or if you already know your Time Machine backup is from a different Mac that's using an earlier Mac OS X version than what came with your Mac) you should use Migration Assistant to move your information over instead of restoring. See the "Restoring your entire system from a backup" section of this article.
With Mac OS X v10.6.3 and later, if you attempt to restore a Time Machine backup that was created on a different Mac, you may receive this alert: "You can't restore this backup because it was created by a different model of Mac". See the "Restoring your entire system from a backup" section of this article.
Important: Even though one version of system software may be identical in version number to another, the software may not be exactly the same. For example, you may have a retail version of the Mac OS X v10.5 installation disc, and your computer perhaps came with Mac OS X v10.5. Your computer may have additional software specific to it included on its software disc (Mac OS X v10.5) that may not be present on the retail Mac OS X v10.5 disc. See this article for more information.
Be sure to use the original software discs that came with your computer when performing a reinstall of Mac OS X unless you are installing a new release (for example, installing the retail version of Mac OS X v10.5 onto a compatible Mac that shipped with Mac OS X 10.4 is fine). This is particularly important if you are creating and installing custom disc images.