RAID 1 issue w/ G-RAID Thunderbolt 3 (8TB)

I used the assistant software (with the help of Disk Util. to initialize) to create a RAID 1 on my G-RAID Thunderbolt 3. (8TB) But I don’t think I did it correctly. “Slot 1” shows that it has a disk. (of which I named "G-RAID 1 Mirror) but Slot 2 says “empty.” Did I do something wrong? NOTE: It does show up on my desktop as 4TB, which of course, is one-half of the 8TB drive.