RAID system with external hardrives
First let's clarify one thing: RAID is not the same as backup. Building a RAID array out of several (presumably USB) external drives would have many drawbacks. See this question I asked a while back for the explanation. I suspect what you really want is passive and seamless disk/data backup in which case OS X's built-in Time Machine works great. And if you have Mountain Lion it is capable of using all of your external drives to store the backups.