There's been a lot of buzz the last few days about Drobo, the "storage robot" from Data Robotics. The best way to get a sense of what it can do is to watch this promotional video. Basically, the Drobo, which has four SATA drive bays, plugs into your Mac via USB and looks to the Mac just like a large USB mass storage device. According to our blog compadres over at Engadget (who have the full skinny), it "uses pooled virtualized storage" (not RAID) to create what looks to your Mac like a single drive, but which has data protection and redundancy features. Best of all, the hard drives are hot swappable. You can add and remove hard drives on the fly without corrupting your data either because of hard drive failure or merely to add more storage space. All of this is transparent to the host computer, which never notices anything has happened.
All of this storage robotics coolness is expensive, however. The Drobo sells for $699 without any drives. You'll need to install at least two SATA hard drives in the Drobo and it handles all the rest.