Not to plug a product, but I am really happy with my Data Robotics Drobo box. Data Robotics sell 4-bay redundant, USB2.0 connected, drive bays. They take standard 3.5" SATA drives. If you have more than one drive in the box, it creates a redundant file system, storing 2 copies of each block in your filesystem. This gives you data redundancy, allows hot-swapping when drives start to fill up, and make sure that your data is secure.

www.drobo.com/

Aside from off-site storage, this drive bay automatically backs up the data on it. While not a cheap solution, it is robust, expandable, and has features to make it worth your while.