Open Firmware is giving up on the boot device before it has a chance
to spin up. (MacOS doesn't have this problem because its boot device
is ROM.) Paul Mackerras gives this fix: Using nvsetenv (from linux) or
Boot Variables, set boot-command to:

begin ['] boot catch 1000 ms cr again

This will make OF retry until the disk has spun up.

Dan Eisenbud points out that the "new improved" Boot Variables will save
this in a settings file so you don't have to enter it every time.

jonh@cs.dartmouth.edu