The Icy Dock MB454SPF-B Backplane Hot Swap Module is popularized by HP MSS machines, these modules have found their into many competitor’s Windows Home Server-specific machines and many home-built machines. It's an aluminum box the size of three stacked 5.25" optical drives which contains up to four 3.5" SATA hard drives. Each drive slides in with rails-only, and is individually hot swappable. At the back of the unit are two power and four SATA data connectors.
Used in the free 5.25" bays of a full tower case or dropped into a standalone external storage chassis, the IcyDock MB454SPF-B provides a convenient way to increase data storage capacity in any computer case with at least three free 5.25" drive bays. It has the added benefit of hot swapping connections, and supports SATAI or SATAII hard drives with SATA power connectors only.
More info: Price $108.99: Amazon