The Western Digital My Book VelociRaptor Duo 2 TB RAID might just be the mother of all external storage devices.

Inside, it packs two WD VelociRaptor 10,000 RPM drives that are user-selectable RAID 0/1 or JBOD. This means you can gain redundancy via mirroring or even more speed with striping. With dual Thunderbolt connectors, you can daisy chain this with other devices easily. Since the whole thing is user-serviceable, you can swap in your own drives if needed in the future.