I'm also new to linux and have been "tasked" to build a cluster. I will probably not be able to help you, so don't get any false hope. Firstly, I take it that you are not running jobs on your headnode? How big is your external usb drive?
How did whoever set up the cluster? Did they use OSCAR or Rocks to help them, or did they do it manually?
Check the sites listed here. Perhaps you could figure out something there:
-look at the list at the end for links.
I don't know enough to help you. Use Google a lot, and try finding some IT geek close to where you work so that he can come over and physically help you.
Hope it goes well!