This has nothing to do with the Zip drive and everything to do with the computer. Booting from a drive means the hardware needs to understand the peripheral bus, independently of the hardware. In this case, Open Firmware, the software that loads Mac OS (or whichever OS you're using) has drivers for FireWire and/or USB drives only on more recent models. Only the current crop of G4s can boot from FireWire devices; similarly, only the iBook and the earlier G4s (as well as the new ones) can boot from USB devices. Some PowerBooks are also included in there; check Apple's new model notes. Hope this clarifies it.