Can't use unallocated space

Good day,

I have a scenario where we just installed Server 2012 with a RAID 5 configuration. We have 3 2TB drives. The OS was installed on a 2TB partition as it cannot install on any larger. Now I have a 3.5TB unallocated partition that I want to convert to GPT partition to be able to use it.

The problem comes in when I right click on the unallocated partition all options are "greyed out" and the only thing I can click is on properties.

Can someone please help me to convert this partition to GPT so I can use it?

Thanks

Rudi