Expanding a Windows partition is not the same as expanding a Virtual Disk. The Virtual Disk is handled by the controller and, once configured, is presented to Windows as a physical "disk" in Windows Disk Management. If there was more than one VD configured on the controller, it would appear as multiple "disks" in DM. Since you only have a single VD, you can expand the VD by adding disks to the array. If you need to expand the actual Windows partitions, that is done differently.