I use a Drobo unit (
Data Robotics, Inc.). It's basically an external RAID array with hot-swappable slots for up to four drives---although they have a unit, now, that accepts up to eight. It works great for me---all my system files are on an internal 10K rpm drive; all my data files are on the Drobo. Although it doesn't, technically, provide redundancy; it spreads the files over multiple drives and I'm comfortable with that. I'd suggest watching the product demo video on their website.
I also have a separate external drive that I use to backup "really important" files.
Jack