Unallocated space is part of disk space that is not belong to any partitions. In Windows Server 2019 native Disk Management console, you can create new volumes with Unallocated space. You can also increase partition size with Unallocated space via Extend Volume function, but Unallocated space can only be added to the left contiguous partition. In addition, Disk Management cannot move Unallocated space, so it is useless in most cases. You'd better run partition editor software to move and merge Unallocated space.
The most common reason why many people want to move Unallocated space is because that Disk Management cannot extend C drive after shrinking D or other partition. As the screenshot shows, Extend Volume is disabled for both C: and E: drive after shrinking D.
As I said above, Extend Volume function can only add Unallocated space to the left contiguous partition. If another Shrink Volume function can make Unallocated space on the left while shrinking D drive, or if Disk Management can move Unallocated space to the left, you can extend C drive easily.
How to move Unallocated space to the left in Windows Server 2019:
If you want to merge Unallocated space to the right contiguous partition E, you can combine with NIUBI directly without moving Unallocated space to the end of disk. To do this, right click E and select "Resize/Move Volume", drag left border towards left in the pop-up window.
After moving Unallocated space from right of D to the left, you can combine Unallocated space to C drive with either Disk Management or NIUBI.
Follow the steps in the video to move and merge Unallocated space in Windows Server 2019:
In few servers, there's not plenty of free space in other partitions on the same disk, some people ask whether it's possible to move Unallocated space from another hard disk. The answer is no. There are 2 options when entire disk is full:
Besides moving and mering Unallocated space in Windows Server 2019, NIUBI Partition Editor helps do many other disk and partition operations.