Not a bootable device.
If you are trying to install Mac OS X 10.3 Panther onto a Beige Power Macintosh G3 using XPostfacto, and XPostfacto keeps insisting that your hard drive partitions are not bootable devices, make sure the OS X partition has been formatted as Mac OS Extended (HFS+).
It’s a simple problem, and mainly derives from an overly generic error message, but it is extremely frustrating nontheless.

You’re still alive! Yay!