I have a modified Seagate Dockstar[1][2] and I'm using it with an external HDD, but I think it would be cheaper using a modified NetGear Stora[3] that has 2 SAS ports than using more than one USB external HDDs (with a dockstar or a Raspberry Pi).

I think more interesting applications are in those field in which you can take advantage of the video HDMI port and the IO ports..

[1] http://ahsoftware.de/dockstar/

[2] http://jeff.doozan.com/debian/

[3] http://www.openstora.com/