The My Book Studio Edition II combines two Western Digital Caviar Green 3TB drives to give a vast 6TB of storage.
Its USB 2 and eSATA connections are joined by a Mac-friendly FireWire 800 port, and the drives are formatted with Apple HFS+ (PC users will have to reformat them). The drives use RAID0 striping (RAID1 mirroring is also supported), but using the USB 2 connection limits read and write speeds to 30MB/sec.
Even over eSATA, write speeds maxed out at 136MB/sec, with read speeds of 106MB/sec – merely equal to a regular internal drive. It’s fairly expensive, so recommended only if you really need this much storage.