Mac OS X v10.6: After installing, Mac still starts up into Mac OS X v10.4 or v10.5

Resolution

Mac OS X v10.6 has the ability to "back out" of an upgrade installation if there is an issue during the installation process.  If this happens, Snow Leopard will leave your Mac in a bootable state with the previous Mac OS X version intact.

An issue with your DVD drive, or an issue with the Snow Leopard Install DVD itself, are the most likely cause of the issue.

Follow these steps:

1. Insert a DVD that you know has no issues; for example, your Mac's original DVD installation disk.
2. Open Disk Utility from your Utilities folder (in the Applications folder).
3. Click the DVD in the devices pane in Disk Utility.
4. Click the First Aid tab, then click "Verify Disk".
5. If the disc does not pass verification, eject it and try another known-good disc. If it does not pass either, consider taking your Mac to an Apple Store or Apple Authorized Service Provider for a service check on your DVD drive.
    
6. If the disc(s) passes the verification in Disk Utility, eject it.
7. Make sure your Snow Leopard installation DVD's shiny side is free from issues such as smudges or scratches.
8. Insert your Snow Leopard installation DVD.
9. Click the Snow Leopard DVD in the devices pane in Disk Utility.
10. Click the First Aid tab, then click "Verify Disk".
11. If the DVD does not pass verification, you may need a replacement disc.
    If it does pass verification, quit Disk Utility, and attempt the Snow Leopard installation again.
     
12. If the issue persists after installation, consider taking your Mac to an Apple Store or Apple Authorized Service Provider.