SBS 2011 - Disk Partition re size

Hi all

i've got an SBS 2011 Standard Server. it has 2x500GB hard drives on raid 1 and another 2 x 1tb drives on raid 1. i want to split the 1tb drive into 2 500gb partitions. i managed to shrink the partition in disk management, however the disk volume size in disk management is still showing it as one whole disk but in Computer the drive is showing as half the size. any idea why its only half reflected the drive shrinkage?

thanks in advance

Andy