the Raid you've got is pretty good,
your computer you have got is not very good.
Using a thunderbolt raid on a USB connection is senseless.
The reason for a Raid system is to gain performance together with redundancy.
In your case you don't get performace from your raid.
You can just plug a Sata Drive to the USB connection and you get as much performance
as from the Thunderbolt raid.
Mac pro 8core
several raid systems
That OWC needs TB2 as a host connector.
The FW800 is for input only and not a host connector.
If you have a PC, get a TB2 PCIe card.
HP makes one and is pretty decent.
Of course we have no idea what PC you have so that could be the holdup.