The PowerBook G4 computer can boot from a USB storage device that follows the USB Mass Storage Class specification.
Class drivers are software components that are able to communicate with many USB devices of a particular kind. If the appropriate class driver is present, any number of compliant devices can be plugged in and start working immediately without the need to install additional software. The Mac OS for the PowerBook G4 computer includes USB Mass Storage Support 2.0, a class driver that supports devices that meet the USB Mass Storage Class specification.