If you are trying to install Mac OS 8 or 9 to a NewWorld Mac, you'll need to make sure you have a matching Mac OS ROM file for your machine. Apple has published a table that shows the Mac OS ROM file that shipped with each Mac and Mac OS version since Mac OS 8.1 on the original bondi blue iMac.

http://support.apple.com/kb/TA22055

Peace,
Drew
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Power to the PowerPC!