Running and installing GNU/Linux on an external USB Drive is not difficult, it's slow.
There are many ways to install Xubuntu. The easiest, that most people ignore, is to download the Xubuntu Intrepid Installer from the torrent server (be patient, it can last a bit) http://torrent.ubuntu.com:6969/
First of all: if it is your first install, BACKUP IMPORTANT DATA!!!
After downloading the CD image, burn it with the Apple Disk Utility at low speed. Start from the CD pressing the C-Key
If you want, you can try to install Xubuntu on you external HD, just to get an idea how it can work. Choose it when the partitioner ask you how you want to install (use the entire HD). Be sure to choose the right HD!!!
When you are finished, reboot and at start up, press the OPT-Key. You should be able to see your new installed Linux.
If you have problems with the video, restart and at the Yaboot (the Powerpc Bootloader) prompt, type:
If you find, as I believe, that it is too slow, you should follow what conal suggests you, partitioning the HD. You don't need to reinstall macOSX. You should resize the partition where MacOSX is using the Installer.
Linux video=ofonly nosplash
Let us know, how it works for you, and if have still questions.