Proliant ML330 adding 2nd disk to array b and create mirror

[DirkG]

Hi, I have an Proliant ML330 G6 with Smart array P410 installed.

Bay 1 & 2 = 2 x 250 GB SATA = Array A and configured as Mirror. Running the OS Win 2008 Server.

Bay 3 = Array B and single disk (1TB SATA) and holding Data.

I have installed an extra disk in slot 4 (Identical disk as in bay3)  and want this disk to be added to Array B and then create a mirror (for extra safety of my Data)

I ran the ACU-tool (Array Config Utility) and see the new added disk as "unassigned" (which seems OK at this point)

(See attached file)

Bud running the wizard to add the "unassigned disk", I can only select Array A and NOT array B.

Why? Where do I make a mistake or is this not possible at all?

Hope to hear from you smart people here and learn.  Thanks in advance for every efford. Regards from Belgium.