Well, here's the thing: There are none.
But what does exist is a USB 3.0 and Thunderbolt RAID, the
drobo 5D - and with the optional
mSATASSD, it will also be quite fast.
You can retrofit USB 3.0 to your Mac Pro with a
$19.99 PCIe card (which will probably come in handy regardless of what you're doing with it later).