It is always the nightmare scenario ie: disk failure! It really sucks when your hard drive cannot be accessed anymore. All of those files that you only wished you had backed up. Well that very thought crossed my mind a while back and almost happened to me. I rebooted my computer a couple of years ago, and nothing happened. Well long story short I was able to get my computer fixed and my data was intact.
With that close call in mind I decided that I really had to start backing up client work that I was doing, and other miscellaneous documentation regarding my contract work. I did this back then with some USB sticks, but last year bought a 160 GB NAS and have been quite happy with it. Though I still have a backup to my backup on my USB sticks it is nice to have peace of mind. I know my clients couldn't care less about my computer problems they just want their work done. With backups close by that is no longer an issue. Any of guys go that route?