Let me tell you, it was one of the scariest things I'd experienced professionally, even though I wouldn't have lost "that much" because I did back up a month ago. BUT. I would have lost the entirety of the edit I just finished, which I'd been working on for the last three weeks. It was my first full edit of House of Ivy and Sorrow, and the thought of going back to my first draft was terrifying.
Usually I have a copy of my work in gmail, either from sending chapters to crit partners or manuscripts to my agent/editor. This is probably why I haven't stressed much about backing up my stuff—I know I have a copy elsewhere that I can access not only on my computer.
This time I didn't. The second draft I've been working my butt off on didn't exist anywhere but my computer (that has since changed), and the thought of losing it sent me into a fit of panic. Not only was I possibly going to lose a month's worth of work, but it was work I had a deadline for. I didn't have TIME to lose it when I'm already on a tight schedule (aka: baby due in about 10 weeks). It would have been a huge problem if I lost that one single file.
Luckily (very, VERY luckily), the end of this scenario is happy, because I happen to be married to a computer guy, who was able to resolve everything by the end of the night. No waiting on pins and needles until Monday morning to take it to a store and find out whether or not I lost all that work (I might have gone crazy.). Nick was even able to repair my computer, which at first we thought was dead. It felt like a miracle, and the first thing I did was back up all my stuff (Then I kissed him, because seriously.).
As I lay in bed last night thinking about it all, a strange realization hit me: I can't be flippant with my backups anymore. This is my JOB now (Such a weird thought!). Every draft I write is potentially something I could sell, and setbacks have bigger consequences than ever.
I know, I'm slow coming to this realization. But I really never thought of it that way! Before having an agent or editor, I wrote a lot of stuff. Sure, I didn't want to lose it, but if I did I had the time to recreate it. The deadlines were all my own, even if I took them seriously. Now I have people expecting my work at certain times. Real, live people who make part of their living off my words. That is a huge responsibility (And kinda scary at times, gotta say). I can't afford to lose a month's worth of work because I am forgetful about backing up my computer.
So get in the habit now. It's not hard to back up files, and you never want to be in a situation where you've lost work. I've certainly been scared straight.