Problem mirroring 2 HDDs

Good day.
I have 2 identical hard drives, ST2000DM001 (2 Tb) which I want to use in a raid 1 mirror configuration. They do NOT contain the system OS, rather as storage with drive designations F: and G:. The OS is Windows 7 Professional and I intend using the OS raid 1 utility.

Here’s my problem, when viewed in the Disk Management, both disks have the “Add Mirror” option greyed out. Both disks are Dynamic, however, one disk show a separate 8 MB Unallocated partition and I have no idea how that has occurred.

Help, anyone?

Regards,
Bjoern