Xiotech’s ISE segregates mounted disk drives back-to-back on a center-plane (as opposed to a backplane). The drives that are back to each other spin in opposite directions — this counters vibration. Additonally, the center-plane acts a heatsink. Reducing drive vibration and heat are two of the integral factors.
Notice also that there is no backplane anywhere inside the 3u enclosure — this means air flows fully through the device which means much more cooling with less energy expended.
Our friends at SUN FISHWORKS have studied this and produced a video to demonstrate how vibration can affect drive performance:
You can read all about it here: http://blogs.sun.com/brendan/entry/unusual_disk_latency