[ubuntu] Ubuntu on externel HD - using iMac

Hi,

After a long time I would try to get Ubuntu again.
I installed it on a External HD but I have no dual boot on my Imac.
At the end of the installation I got an error,no startup on the disk,or something like that.
What can I do to start ubuntu from external HD?
I try to use refit,but don't understand it good.

Kind regards.