iMac G 5 PPC

place the dvd in the dvd drive. may have to power on machine then insert dvd.

power down mac

Try holding down the option key then power on. This brings up the startup manager. Click on your dvd icon. Click on right arrow key.

This will boot up install program. May need to format the hd.

here is how.

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Format a disk using the installation DVD

To format the startup drive, you will need to run disk utility from your installation DVD.

This article will tell you how to get to disk utility. Once in a disk utility, you can go and format the disk.

http://support.apple.com/kb/TS1417

To format your startup drive, you will need to run disk utility from your startup DVD.
Mac OS X 10.4: About the utilities available on the Mac OS X 10.4 Install DVD

http://support.apple.com/kb/HT2055

How to run disk utility from your startup DVD.

1. Insert your startup DVD into your reader. Power down your machine. Hold down to the c key. Power on your machine. This will bootup your startup DVD. ( Alternatively, you may hold down the control key, this brings up the startup manager. Click on the desired volume. Click on the right arrow. )
2. This will bring you to a panel asking you for your language. Pick your language.
3. 
   
4. You you come to the Install Mac OS panel. Do not install.
5. Click on Utilities menu item. This will give you a pulldown list of utilities.
   
6. Click on the disk utility.

7. Start up disk utility.
   On the left pane view, you will see a list of all your disks. Click on the external disk.
   Click on the partition tab.

   

   
   

8. You will now see how your external disk is currently set up. Fill in the information as appropriate. You should pick Mac OS Extended (Journaled).

More details on formatting.
http://www.kenstone.net/fcp_homepage/partitioning_tiger.html