unallocate space Windows 8

a little lost

Honorable
Jul 30, 2013
1
0
10,510
I have just finish building a computer. The 3TB Seagate hard drive as 745 GB worth of unallocated space that I cannot seem to use. I try "managing" it and it doesn't allow me to extent my existing 2TB partition to the full space of the drive.

My operating system is install on 2 128GB in RAID0

Is anyone able to help me recover those 745GB of space?