When you have an old PowerPC Mac but no OS DVD, you have a problem. You cannot boot from USB as these old PowerPCs only support booting from internal disks, cds or via firewire.
There is however a way to convince your old Apple’s OpenFirmware, that your usb drive is actually a cd-drive and here is how to do it.
To enter OpenFirmware hold down Command-Option-O-F.
Now in order to make this work, you need to find out which usb device your thumb drive is mapping to.
0 > devalias [...] usb0 /pci@f2000000/@15 usb1 /pci@f2000000/@15,1 usb2 /pci@f2000000/@15,2 hd /pci@f4000000/ata-6@d/disk@0 [...]
Now we know how to address our usb drive. Let’s map it to the alias “cd”. And next load the bootloader.
0 > devalias cd /pci@f2000000/usb@15/disk@1 0 > boot cd:,\System\Library\CoreServices\BootX
In order to test if your alias is working, you can run this command and let open firmware show you a directory listing:
0 > dir cd:,\ \