Do It Yourself NAS

The Drobo FS is one of the first entry-level NAS systems to include 5-bays, most products in this class only support 4-bays. This means that the maximum capacity of the Drobo FS is 10TB, compared to 8TB in four-bay NAS units.

via StorageReview.com.

StorageNerve has benchmarks for the 4-slot Drobo connected using USB 2.0 and Firewire 800.

Toms Hardware reviews the Drobo USB DAS device.