You didn't do too much SEARCHING, did you?
Whatcha did was install grub (the bootloader) to your external drive. Now none of your operating systems know how to boot when the drive isn't there. If you had spent more time (literally, seconds) taking a look into this problem, you would find that it's a quick, simple fix. Although this tut is for grub 1.5, it still applies to grub 2.0, which is used with BT4. And guess what? It was the first page I came across with my four-word search.
Since I don't want anybody else confused, I'm rewriting this tut a wee bit:
Boot into the original Linux system with the external hard drive still installed (or you can boot from a Live Ubuntu CD).
Open up a terminal window (Konsole)
Type fdisk -l and locate your Linux boot partition from the list Example: sda1 or hda1
Type grub-install /dev/sdx or grub-install /dev/hdx to reinstall or repair Grub!
Reboot and test!
Notes: x represents the drive letter a, b, c, d etc. Replace with your actual drive letter.