That darn reoccuring "unable to boot' question

[Dan.y]

The preference panel shows up the internal OSX, the internal 9.2, the external (on the Western Digital) 9.2 but not the copied OSX on the external.

It shows only internal OSX and the external 9.2 y when I hold down the 'Option' key when starting. In either case the 9.2 doesn't start the computer from the external drive.( the internal 9.2 does boot the computer). The OSX as a copy is not seen.

I would try booting the computer from an OSX disk but my disk drive is broken. (its an older black 500 firewire powerbook) was hoping to wait a few months before upgrading.

Is the Western Digital compatible with the Mac for booting or is there a possible other factor in the mix?