A couple of questions.
First, do you need / want / have license to use the files on those drives, or can they be scratched? Second, do you want to use the features of the GRaid box, or are you happy with just two new empty drives?
I ask because, depending on the answer, the process can be simple or complex. If you just want to use the drives as new drives, I would go the simple route of re-initializing them as GPT drives (or even plain old MBR drives), creating one partition each, and formatting the partition. It would be like you got two new drives.
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If you need to read what's already on the drives, you could download some software to read ext4 from Windows - google "ext4 file system windows" and all sorts of stuff will pop up.
If you don't want to give up the external functionality, such as portability and possibly using RAID1 to enhance reliability (at the cost of half of the usable storage space), then you need a way to connect that box to your machine. Does it have any ports other than Thunderbolt, such as USB 3.0 or SCSI over Fiber Ethernet?
(note that the latter choice was a bad joke)
EDIT: If this is the product:
http://www.bhphotovideo.com/bnh/controller/home?O=&sku=1024614&gclid=CI6J5fydhcUCFZIdgQodWVgA8g&Q=&is=REG&A=details , then it has USB 3.0 and you can just plug it into a USB 3.0 port. Heck, you can plug it into a USB 2.0 port, but that will limit the speed.