Booting Mac OS 9 from a USB flash drive 

To be able to boot Mac OS 9 from a flash drive, you have to copy your System folder and any other software you might need to the flash drive.

To boot the machine, first reboot using the Mac OS install disk, by holding down the “C” key as usual. Then select your Flash drive in control panels> startup disk. If you choose the flash drive from Mac OS X, then it doesn’t boot from it.