Verifying mirrored data in My Book Thunderbolt Duo RAID 1 mirror mode

Is there any way to confirm that the My Book Duo is duplicating the data from one drive to the other without removing the drives? I have it in RAID 1 mirror mode. I can’t see the data on the second drive (or even see a second drive) therefore I’m nervous since I can’t visually confirm its existence.

There is no direct method to manually access the second drive since both drives working in tandem as 1 drive in RAID 1 (Mirror). If the RAID volume is healthy (Not degraded) it is certain the data has been copied since it happens at once for both units.