Try these troubleshooting tips one at a time until you are able to start from the disc:
- Verify that the shiny side of the disc is clean (no particles or smudges on it). Here are some tips for cleaning a disc.
- Make sure you are using the disc that came with the computer, or a newer disc that contains a compatible version of Mac OS X (look at version on the label of the DVD). Please see:
- To verify that your Mac is attempting to start from the disc, hold the C key as your Mac starts up. You may hear access sounds from your computer's optical drive.
- If holding C does not help, hold the Option key during startup. Startup Manager should appear. Select your Mac OS X disc icon, then press Return.
- If Startup Manager does not appear, make sure you are using an external Apple keyboard (directly connected), or a portable Mac's built-in keyboard. Some third-party keyboards may not be recognized in time for Startup Manager to appear.
- If you are able to start from your hard disk, you can use Startup Disk preferences in System Preferences to select the disc, then restart.
- Temporarily disconnect USB and FireWire devices connected to your computer. If you are not using a portable Mac, connect your Apple keyboard and mouse directly to your Mac. If this resolves the issue, reconnect USB and FireWire devices one a time until the issue occurs again. Once a problematic device is located, contact the manufacturer for assistance.
- Your Mac may benefit from a firmware update. The latest firmware updates are available from Apple Support Downloads.
- If the issue persists, shut down your Mac, then remove third-party hardware upgrades such as third-party memory (RAM) and third-party expansion cards. If this resolves the issue, contact the manufacturer of the product for assistance.