To boot a PowerMac (or an iMac, iBook, eMac, etc...) from an USB stick or drive, if you have a G5 or late G4 model, first refer to the boot from CD guide and if it does not work, then follow these instructions:

1. Plug your USB stick or drive in an USB port

2. Power the computer on (or restart it if it's already running) and immediately...

3. ...hold down those 4 keys together: [ CMD + OPT/ALT + O + F ] until you see the white Open Firmware screen similar to this:

4. Then at the Open Firmware command prompt, type this and press ENTER:

(Please make sure that there are 2 backslashes before :tbxi)

Note: You may have to change usb0 for usb1 depending on which port you plugged your USB stick in.  If it still doesn't work, you can try this command:

Note 2: If you want to boot a non-Macintosh partition, say for instance a PPC Linux distribution, then change tbxi for yaboot

Note 3: If you get a CAN'T OPEN error, it means that you forgot to plug in your USB stick BEFORE powering on the computer, so turn the Mac off and start over from step #1.

Finally, sometimes it just doesn't work, no matter what.  It could be that the disk image you cloned was not correctly dumped to begin with and it is lacking a tiny partition or a boot sector that prevents your Mac from booting it today, or it could be that your USB ports are not behaving correctly.  Know that if this process fails, there is another method which will require you to temporarily borrow an old hard drive to launch the Mac OS X installer.