I would wager you have the drive using the old MBR partitioning scheme. MBR has a limit of 4 partitions which is probably why you can't create another. If the system has been imaged I would do a re-install of Windows and select the advanced option. Select each partition and delete them. It should then create the nessicary partitions and extend the OS partition with the remaining space. You can manually move partitions but it can be quite difficult, takes a long time and requires special tools to be run in an offline mode.