Windows 2000 svr and Disk partitioning

I have a Compaq Prolient windows 2000 server.
3 X 18.2GB hard disks running Raid 5
I have created 3 primary NTFS partitions configured as follows: C: 4GB E:5GB F:5GB
I have 20 GB unpartitioned space.
My question is why can I not create a partition out of the 20GB free space.