A couple days ago, the IT guys at work had me drop off my laptop for reformatting. Although I had told them none of my files were backed up and they had said everything would be transferred to my new hard drive, when my laptop was returned, all my files were gone. Of course I had a minor nervous breakdown over all the work I was potentially in for in recreating everything, but my biggest comfort was that a backup copy of my WIP was resting comfortably on our desktop. I would have cried big, salty tears if I'd thought all of that effort was down the drain.
There is a happy ending on the work front also. Luckily, they had not wiped my old hard drive and were able to transfer all my files to the new drive.
So, the moral of this story is: save yourself some heartburn and back-up your writing regularly!