honest question: what's the best way to back up your data?
Well this does depend on what you want to back up.
I personally don't think anything will kill you except losing your notes/papers. Those all together even conservatively shouldn't be more than 100 MB or so (and that's very conservative, most documents won't hit 1 MB unless you add pictures etc. to them).
In this case, I would highly recommend a USB drive, these are hard to break unless you run a car over them, people keep them on their keychain and they get banged up all day and never fail. A 1 GB USB drive should do the trick.
Gmail is another option, although I haven't seen a program that uses it like a harddrive that seems to work well, and filing stuff won't be as easy as filing them in folders on a USB drive.
The external is good if you want to backup the WHOLE system, but #1. I don't think it's necessary and #2. external drives in my experience have been more unreliable than internal harddrives, so I still would back up your important stuff on a USB drive.
Personally, I think I'll backup the important stuff on the USB drive and store it on my desktop, but I have never had a harddrive go bad on me on a desktop, I would almost feel safe just keeping a copy on my desktop, but it's important enough (at least notes/outlines) that backing it up is worth the extra few minutes.