Boot a PowerPC Apple from USB

Booting from USB for PowerPC (PPC) iMac's is problematic to say the least. In any case, my 1.6ghz G5 Open Firmware version 5.2.2f4 won't support it without getting acquanted with Open Firmware. After some serious digging around and rather a lot of experimenting I found a way though:

  1. Be sure to partition the disk with an Apple Partition Map (i.e. not GUID or MBR)
  2. Determine the partition where your bootable image it situated (e.g. an MacOSX DVD or DMG restored to a partion with Disk utility's restore). This might be disk1s3 in which case the partition number is 3
  3. Restart your iMac while holding down Command-Option-O-F (Alt-Cmd-O-F). This will land you in Open Firmware.
  4. Type:
    dev / ls
    to get the device tree/list.
    Look for something in the output like:
    /usb@b
      /disk@1
    
    As we're talking about a tree here, write down the complete path to this node. In my case it would be:
    /ht/pci@2/usb@b/disk@1
  5. Type:
    devalias ud /ht/pci@2/usb@b/disk@1
    In other words: make 'ud' equal to the path you found in step 4.
  6. Now verify you got the right disk:
    dir ud:3,\
    (3 is the partition number you wrote down in step 2)
    And look for a file with tbxi attribute, probably in:
    \System\Library\CoreServices\BootX, e.g.:
    dir ud:3,\System\Library\CoreServices
  7. Then boot from it:
    boot ud:3,\System\Library\CoreServices\BootX
  8. Presto !
    USB's your uncle.
Albert Siersema
Fri Apr  2 17:54:22 CEST 2010