I store data on my computer in RAID1. This data is backed up on a regular basis on two separate external HDDs stored in 2 separate locations.
When selecting an external enclosure, choose one that has a fan built-in. Like this one: http://www.newegg.com/Product/Product.aspx?Item=N82E16817173042
It is rugged, well built and has an eSATA connector as well as USB 2.0 - they also have a similar model with USB 3.0