The failure of a hard drive is a nasty business, especially if you don't have a verbatim copy of your drive backed up elsewhere. Even if your critical data is backed up elsewhere, it is still a pain to set up a system again. I recently went through a drive failure that was gradual rather than sudden, and still barely escaped without losing some of my most critical files. I am currently studying outside of the US, so I only had partial backups and when things started going south I had what I gradually realized was a race against the clock to save as much as I could. Of course, I made a few mistakes along the way...
http://arstechnica.com/articles/culture/hdfailure.ars