I was piecing something like this together on my own with a laptop and a pile of external hard drives, when I found my desk overrun by the little things. They were too fragile and didn't stack well. I started looking around for a shelf to hold external hard drives and do cable routing, when it hit me "oh yeah, that's called a desktop computer." So I bought a nice computer case and a relatively inexpensive motherboard/CPU combo and filled it with disks. Way sturdier, and much faster. I'm not sure at all what I was thinking before with that USB idea.

Using external hard drives as a semi-permanent storage array is not a good solution. For an appliance, Drobo/Synology has the right idea here, and I think this group is making a mistake by expecting people to build a disk array out of a hub and USB drives.