Diskpart.exe is a text-mode command interpreter that enables you to manage objects (disks, partitions, or volumes) by using scripts or direct input from a command prompt. Before you can use diskpart.exe commands on a disk, partition, or volume, you must first list and then select the object to give it focus. When an object has focus, any Diskpart.exe commands that you type act on that object.
You can only give focus to a partition on the selected disk. When a partition has focus, the related volume (if any) also has focus. When a volume has focus, the related disk and partition also have focus if the volume maps to a single specific partition. If this is not the case, then focus on the disk and partition is lost.
Disk Management Tool
Type diskpart to start the utility.
Type list to get an overview over all disks and volumes.
Select the volume you want to work with i.e. select volume 1.
Once this volume is in focus, you can type “extend” to use all available space.
Note on extending a system partition:
The system volume can only be extended by running Diskpart.exe from a command line by bootable CD.
To extend system partition on Windows Server 2003, there must be some unallocated space behind the system partition. Otherwise, you cannot extend system partition with diskpart command, so a third-party server partition software is needed.