One thing that I would caution is do not think that one data backup to another HDD, offsite, or not is going to be enough redundancy for you if your data is truly that valueable to you. I would suggest backing up to DVD-Ram, RW, R discs also, or for added redundancy. one rule of thumb is if you do not want to lose it. back it up everywhere!
I personally have all of my data backed up on 3 HDD's as well as on CD-rw discs(wish I had DVD-Ram). that way I can restore from anyone of those sources. you never know when a HDD is going to fail regardless of how little you use it. it is a machine like any other with moving parts that can easily be damaged by alot of different factors such as Physical shock(from dropping), to Electrostatic discharge to a factory defect(maybe a microscopic fleck of dust went through the mfr filters, and deposited on the HDD platter). The point is you never know when a failure will happen, but guaranteed it will happen at the worst time...