Triple boot Ubuntu, Windows and Mac problem

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http://askubuntu.com – I run on an intel core 2 duo late 2009 imac 21.5" I have rEFIt installed which changes boot camp (efi or w.e) to show a triple os boot, mac, windows, and linux. I want to install to my WD Elements 2TB external HDD but it never works, i've been through some techniques in the installation to get it to work. So.. i install boot loader (grub) to sdb like i should, sdb has root, boot, and swap partitions (as well as free space).. dw boot has at least 258mb. When i reach the end of the setup where it tells me to restart my computer i hit that, then it comes to a black screen showing logging info (HowTos)