The Power Mac G4 Cube can boot from a USB storage device that follows the USB Mass Storage Class specification. For information about the USB specifications, see the references in USB Interface.
The Power Mac G4 Cube can boot from a FireWire storage device that implements SBP-2 (Serial Bus Protocol) with the RBC (reduced block commands) command set. For information about the FireWire specifications, see the references in FireWire Interface.