External HDs are great, but if you're really worried about your files you should just consider an online backup service.
Carbonite is extremely easy to use, the automatic file selection works for most people (unless you save to non standard directories, like the C drive root.) It's about $60 a year. This is what I usually put my non-technical clients on, just cause of ease of use and easy to check status.
I'm sure there's probably cheaper alternatives out there too; I usually deal with "ease of use over cost" type clients though.
BTW the reasoning behind online backup over external is that external doesn't really cover you from theft and fire. As much as you don't want that to happen, it does. And if it does your external will be gone too. (so it comes down to how much your files are worth)