I have two, but I'm just running them as normal. Would love to have them in RAID0 but I have about 350GB of data that I would need to back up and in addition to the 640 drives I only have one 250 GB to use! Might get a third and use that as a pure backup drive and keep the 250Gb for emergencies, saying that though I'd be too tempted to have three in RAID0!