Problem installing/booting Kali Linux on external FireWire HDD

Hi everyone,
I'm not sure if this is the right spot for this (I'm new to these forums), but I was hoping to get some help with this issue. I have a MacBook Pro 8,2 currently set to dual boot Windows and OS X. I also have a 1TB external FireWire HDD that I use for general storage and backups.

I think I've gone through the right steps to install Kali to the external drive:

I shrunk my existing partitions to leave me ~50GB of free space

Used the Kali Live CD to repartition the space (made boot, root, and swap partitions)

Installed GRUB to the external HDD (/dev/sdb)

However, when I reboot my machine, I cannot boot to the newly created system. When I hold down 'option', I am greeted with my usual choice of OS X, Windows, Recovery, Time Machine, and another "Windows" option. Assuming this is just a naming goof by Apple's bootloader, I chose the last one thinking it was my new system. Instead I actually am thrown into to Windows, and not Kali or GRUB.

Here's some more info about my existing setup:

Kali Version: 1.0.3 amd64 Gnome

Mac OS X 10.8.4 setup for Bootcamp with Windows 8 (300GB OS X/200GB Windows)

1TB external HDD partitioned as such (500GB for data (HFS+), 400GB for Time Machine (HFS+), 50 GB of free space for Kali, 50GB for Data (FAT32))

Someone tried to do something similar and wrote a guide but so far I haven't had much luck with it (I'm not knowledgeable enough about EFI booting to trust myself messing with GRUB or my bootloader)
http://www.64k-tec.de/2010/04/instal...e-macbook-pro/

I don't really know where I went wrong, so any help would be appreciated. Thanks!