I have been trying and failing to boot any OS from an external USB HDD on my MacBook Pro Core 2 Duo and have been seeking help in the Ubuntu forums (see http://ubuntuforums.org/showthread.php?t=510030 - I'm lotus49 BTW).

Cyberdork33 pointed me in the direction of this list and quoted the following that was posted recently:

Last weekend I installed Ubuntu 7.10 on an external harddrive attached
to my MacBookPro via USB.

- First installed rEFIt.
- Then installed Ubuntu to the external drive WITHOUT installing grub.
- Note the device name, /dev/sdb in may case.
- Then installed grub manually, using the device name from above.

After that the Ubuntu installation shows up as a boot option in rEFIt.

Hope this helps,

I would love to know exactly what sort of a MBP this was.  If you read through the (rather long) thread you will see that I have referred to other posts on various blogs/forums where people have stated that they have also been successful booting either XP or Ubuntu from a USB HDD on either a MacBook or a MacBook Pro Core Duo, but so far I have not found any evidence that anyone has managed this on a MBP C2D.

Zoltan, if you are reading this, can you let me know exactly which MBP you have and if it's the C2D version, what firmware version are you running?