I've always viewed Raid as being the ultimate answer to this problem. Have two disks in a Raid 1 configuration. If one fails, replace it, re-sync it with the other disk, and you are back in business. With today's computing power and speed of disks it's highly unlikely you would notice performace degradation, even if you use software raid.
But as SD says, it if can happen it will, so while it's unlikely that both disk fail at the same time, it's still vital to backup your personal documents.
Regards