External FireWire-HD/internal CD won't boot

[Stevie]

Hello,

I have got the problem, that my external (mirrored) HD (FireWire) and 
my DVD-drive intern can not be choosed as startup-drives.

This means: they can be choosed, but the cube will not accept them 
when booting after that.
After a short time of search, the system of the internal HD will boot.
I used the original system-CD and apple care CD for the cube.

On the external FireWire-HD (tested with Norton Disk Doctor as OK) 
the cube will start, but after showing the litte Mac it freezes.
This was under 9.2.2, but I also can't start from the OS-X Partition 
on the external drive.

All this was done successfully in the past!

Any ideas what I can try or what might be the reason?
Thanks in advance

stevie