Hello and Welcome to Apple Discussions.
Only the early iMac G4's need a firmware patch for their Optical Drives.
If your hard disk isn't working it's almost certainly not a firmware issue but something else. If you post back the actuall symptoms we may be able to help.
You've also posted in the iMac G4 (aka Flat Panel) forum but your specs say iMac G5. It's worth clarifying which you have as the advice will differ.
Sorry for mix up. Thought the forum was all iMacs.
Symptoms, gray screen or flashing folder. Using various SW tools on CD boot, everything checks out OK (Apple Hardware Test) or OK but no volumes (TechTool, DIsk Warrior). After multiple restarts and trys, changed out HD but still no go. Got error messages a few times (cursor screen) that seemed to indicate firmware problems, but this is not reproduceable.
Did the hard resets. Tried to reinstall OS X but no volume to install on.
Any suggestions beyond take it to the shop?
I can boot from a varity of CD or DVDs (OS X install, Tech Tools, Disk Warrior, Apple Hardware Test). But the internal HD does not show up as a volume. Replaced old HD with new drive but no change. Does not mount, can't see it to mount it with any of the above tools).
I did not think it was possible to boot from a FW drive. Actually I didn't think you could install OS X on an external drive. If so this could fix my problem.
I think you need to take this in for a professional to look at. I suspect if could be an issue with the SATA bus on the logic board.
Both these things are possible.
An external firewire harddrive could well be the most cost effective solution to this issue. Just boot from an Install disc and install OS X on the external drive.