IcyDock makes a 4 port chassis for SATA drives that allows you to build your own RAID out of large and inexpensive drives. The resultant JBOD can then be formatted into RAID0 or RAID1 (software RAID) and presented to backup applications (ie – Retrospect) as offline storage. Amazon sells an IcyDock, populated with 1.5TB drives for a total of 6TB, which is how I’m now snapshotting my VMs in my lab. I’m also using it as the backup destination for my home Kerio server. Works nicely so far.
You can also buy the IcyDock with no drives and likely populate them with 2TB drives, although I haven’t tested this yet (aka – requires confirmation). The IcyDock connects to Mac, Windows and Linux machines over eSATA and the drive hot swappable modules are eSATA. If you don’t already have an eSATA card for your Mac then then you can get one of those at Amazon as well. If you would rather roll with the 2TB drives then you can get those at Amazon too!