Boot the powerpc with usb disk

1. Use Option-Command-o-f boot into open firmware

2. Use devalias to find device list

3. Enumerate the usb disk device number

dev usb0 ls
dev usb1 ls

4. Check whether the disk is readable

dir usb0/disk@1,\\:tbxi
dir usb0/disk@1,\\:tbxi
dir usb0/disk@1:1,\\:tbxi
dir usb0/disk@1:2,\\:tbxi
dir usb0/disk:2,\\:tbxi

5. Boot the system

boot usb0/disk@1,\\:tbxi

See also
http://en.gentoo-wiki.com/wiki/LiveUSB_on_PPC