Grub error installing to USB HDD

Hey folks
Hopefully someone else has come across this problem. I have installed 10.4 (numerous times) to a 1TB External HDD and each time I tried to boot all I get is black screen, white letters that say GRUB GRUB GRUB GRUB GRUB GRUB repeatedly across the screen and never gets beyond that. I have tried doing the install using the various options as to where to install Grub from the last screen of the set up overview (advanced options) and none seem to work. Odd as I had done this some time ago and can't remember how on an older external drive. Ideally, I'm trying to use my macbook pro as the system but Ubuntu running off the 1TB drive as I am running out of disk space on the Macbooks drive and would prefer not to have to do a complete reinstall or having to fiddle with Bootcamp and refit again, (have gone that route before).

Any ideas?

Thanks in advance
Jim