Extend C Drive Partition Failed --- Raid 1

My new server running WSE 2012 came with a 80gb C partition, and the rest of the 500gb disk in the D partition.  The 500gb disk is a raid 1 volume.

I deleted the D partition and tried to extend the C partition but the extension failed.  The error message

says that it was not an allowed size.

Any ideas how to get a larger C partition?

Thanks.