So whats the point of having the backup software if you can't fully utilize it. Backing something up is only 30% of the battle if you ask me. 70% of it is verifying the backup scheme you have implemented works by performing a restore. I say 70% because verifying by restoring also denotes you know how to restore (and know where it is backing up to..). Simple as it sounds, how often does someone perform a restore? Not very often, so it is fairly easy to forget.
Second if you want to get a little more fancy and buy a third drive you could implement RAID 5 which would allow one or more, if you add more drives, drive to fail and the system would still stay running. Again this won't give you the option of restoring a system to a clean image in the event of a virus, like an external media back up solution could.
As others have said though, there really is no way around doing a backup. I saw someone recommend Online Backup, which has its place, but they are prone to crashes, failures, and security risks too. Unless they have some kind of insurance policy to give you some sort of compensation in the event of something bad happening. Even then I don't trust them that much.