Trying to boot debian installed on external HD through rEFIt

[CannonBallGuy]

Hi,

Installed debian onto an external HD and am now looking to use rEFIt to boot from it.
I'm on a 20" iMac with a 2GHz Intel Core Duo processor with OS 10.4.8.

When selecting the debian drive in the rEFIt menu I get the following documented error:
http://refit.sourceforge.net/doc/c4s1_notfound.html
However, I actually have a more recent version of the firmware, specifically IM41.0055.B08, but still get the error outlined at the link above.

Does anyone have some idea as to how I can fix this?

I've spoken to some computer geeks about this and one non-mac user suggested that I need to update my mac's Master Boot Record.
Is this likely and if so, how can I do it?

Thanks,
CBG.