1. Your Mac starts up using a Tiger or Leopard installation
“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.”
2. Not being able to install Snow Leopard on a specific drive
1. Start up from the disk you want to install Snow Leopard on.
2. Move the "Backups.backupd" folder from the root level of your hard drive to the Trash. Enter your admin password if prompted. Important: Do not empty the Trash unless you have a backup on a different disk and want to delete the Time Machine backups from this disk.
3. Move the "Backups.backupd" folder from the Trash to the desktop.
4. Install Snow Leopard.”
3. After installing Snow Leopard, you get a gray screen
According to Apple, getting out of this situation is really easy:
“- If you have already upgraded to Mac OS X v10.6 and are experiencing this issue, reinstall Mac OS X v10.6.
- If you haven't yet upgraded from Mac OS X v10.5, finish any incomplete software update installation by restarting your Mac. Once your Mac has restarted, you can proceed with upgrading to Mac OS X v10.6 Snow Leopard.”
