Booting PowerPC Macs from USB

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Before starting

Not all PowerPC Macs that have USB ports support booting from USB. My Revision C tray-loading iMac G3, for instance, does not. Your USB mass storage device will not have a /disk@# (where # is an integer) node in the device tree if USB booting is unsupported.

Booting may be slow. If your Mac only supports USB 1.1, the fastest transfer speed you will get is 1.5 MB/s.

Some operating systems might refuse to boot from USB, even when doing the tricks listed below. I can't get the Mac OS X 10.2 (6C115) installer to boot past the "Waiting on root device" phase on any of my Macs when booting from USB.

Simple booting

The USB mass storage device may not appear as a destination in Startup Disk. It might not show in the pre-boot device selection when you hold Option (if your Mac supports that). You will need to enter Open Firmware with Command-Option-O-F. This will bring you to a command-line interface. If your keyboard doesn't work here, try a different one; my wireless keyboard doesn't work here on most of my Macs.

In Open Firmware, the hardware in your Mac is arranged in a device tree. Paths in this device tree can become long, so there are some predefined aliases, such as cd. Aliases for USB ports should also be defined. To see these, take note of which aliases start with usb after entering devalias.

To find your USB mass storage device, you will need to inspect the tree of devices attached to each USB port. To do this, enter dev usb0 ls, replacing usb0 with the port you are currently looking at.

If you see a /disk@8 (or some other integer) node in the tree, this is your mass storage device. To boot, enter boot usb0/disk@8,\\:tbxi, substituting names as necessary.

This will attempt to boot the Mac by searching usb0/disk@8 for a file with type tbxi (Toolbox Image). If this does not work, see the advanced section below.

Advanced booting

You might need to trick the OS installer into seeing the USB mass storage device as an optical disc. I need to do this to get the Mac OS X 10.5 Leopard installer to boot. Additionally, you might need to boot in a different way.

  1. Find the full path to your USB mass storage device in the device tree. dev / ls
  2. Replace the cd alias with this path. devalias cd /path/to/device/disk@#
  3. Set the boot device. setenv boot-device cd:,\\:tbxi
  4. Optionally, enable verbose logging. setenv boot-args -v
  5. Boot in another way. mac-boot