yaboot error on ppc instalation on external HD

[altern]

hi

i am not sure if anyone has already talked about this issue.
I am trying to install ubuntu on a powerbook 12". I am using an external 
usb 160G HD and i am trying to install ububtu there (i am not confident 
yet to install it on my main HD).
I get an error saying that yaboot cannot be installed. Then it asks me 
to say if it should be installed on /dev/sda2 but egain it cannot 
install it. So i continue without installing yaboot and it says i should 
run Ubuntu with root=/dev/sda3 .
i continue installing and the machine restarts. Then it wont load from 
the external HD, i guess it needs yaboot.

i dont know what i should do. I dont understand very well this 
partitioning business.

-- 
enrike