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: boot usb0/disk@1:,\\:tbxi

(Please make sure that there are 2 backslashes before :tbxi just in case this page contents gets screwed up in the future)

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: boot ud:,\\:tbxi

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.