I have a partial solution but it is not a whole one. I have an old Lacie portable drive that is similar to this new model Lacie "portable" Hard drive. I connect it to my late 2009 i5 quad core iMac. That machine runs USB 2.0 and Firewire 800. I bought the drive a few years prior when we used to "Daisy Chain" drives together. Here's the cool thing about that... the back of the portable Lacie Drive has 2 Firewire 800 ports, a Firewire 400 port, AND a USB port. All for connecting to the computer and "daisy chaining" other portable drives to it and thus adding more storage. It's old school methodology now. But I love this drive because it allows me to connect my "FirePod" audio interface using the Firewire 400 port. This is really amazing when you think about it. Using a portable drive to connect audio hardware and transition from Firewire 400 to 800 to do it.
I can also add USB drives and connect them although the cable has to be the "square shaped end" not the thin rectangular connection. You know the one usually on the back of a USB printer. The only problem is that this old drive was manufactured with USB 2.0. But if you can find an old hard drive with all of those ports in the back that is USB 3.0 and Firewire 800 then you should theoretically have an interface that transmits your USB connected drive to your Computer Via Firewire.
Well low and behold as I was looking for a picture of the rear end of my old drive I found the newer model that looks like it's got it all. Right HERE. That should do the trick and I think I may just buy one myself so I can interface my new USB 3.0 Raid to my old iMac. Fortunately my MPB has USB 3.0 so no worries there.
This should do the trick and you don't have to buy a computer to put in the middle as was suggested previously.