Turn off the target computer with the USB inserted.Running the Installer Through Open Firmware, the Easy Way If you do not see the bootable USB and only see the Macintosh HD drive, or the installer doesn't run, you'll need to run the installer through Open Firmware - turn your Mac off and familiarize yourself with the instructions below.Click on the bootable USB, and it should run the installer. The fans will go crazy and you'll see a screen with two options: The Macintosh HD and your bootable USB. Once you hear the startup chime, press and hold Option.If the Mac doesn't run the installer and instead brings you to the login screen, restart the Mac.If the Mac boots into the installer, you're all good and you can set up the Mac normally. Eject all other devices connected to the machine, save for the mouse/keyboard and the bootable USB.If you are planning to restore a machine different from the one you created the USB on, eject the USB from the computer and insert it into the machine you will be restoring.If the machine it is inserted to is the machine you plan to restore, keep it in and do not eject it.If it still doesn't say disk1 that's okay, it just might be a little more complicated to deal with later. Perform the same process and ensure that the Disk Identifier is disk1. Then, insert the bootable USB again and open Disk Utility. If it doesn't, close Disk Utility and eject/remove all other devices connected to the Mac except for the keyboard/mouse. For the majority of people, it will say "disk1". Next, look for an entry named something similar to "Disk Identifier".Look for an entry labeled Partition Number. Press Command+I to open the details/properties menu. This will come in handy if you need to run the installer through Open Firmware. Now, it's time to take some notes on the disk. The bootable USB should be complete at this point.Either sit right next to it and move the mouse occasionally, or change the sleep settings in Energy Saver.
Wait for it to complete and do not let the Mac lock your profile, run the screensaver, or go to sleep.
The first mirror server will likely provide the fastest download speed.
If something's not clear please feel free to ask me. I had to combine steps from multiple guides to finally get the Mac to boot from the USB, and figured that I'd post a comprehensive guide here. I had followed many guides online but was never able to get the Mac to boot from the USB or install OS X. The iMac already came with OS X 10.5.8 installed but I wanted to do a clean install of the OS for security/performance purposes. So this all started when I got an old iMac G5 iSight model from a computer recycling store.
While this guide is targeted toward Mac OS X 10.5 Leopard, it still applies to other versions of Mac OS X, including any Server versions from 10.4-10.5, and possibly 10.0-10.3 as well (although I haven’t tested that).
I figured I would revive this subreddit with a working guide on how to create an OS X Leopard bootable USB for PowerPC models and actually boot the computer from it, and provide some extra tips/software suggestions.