Install Fedora on PPC (PowerPC) Mac
As SuperDuper copies files to the usb device, the dialog will show, near the end of the backup, that it is making the device bootable. After the files are copied, you can check that it has completed this step by checking the log on SuperDuper (Window --> show log).
Make sure you have copied all the files from your hard drive to the usb drive. Then with the usb drive plugged in, reboot holding down the "option" key? You should see the usb drive as a bootable device, and selecting it will alow your machine to boot from it.
I know this works, because I have done it many times. If it doesn't work for you, try moving the cable to another usb port on your computer.