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..
I had one of those. It's a very limited device, it runs some custom embedded OS (certainly not Linux) and craps out if you try to access it from two workstations at the same time, etc. etc.
At around $50, might be cheaper to go with this. Unless you are implementing the NAS just because it can be done :-)