Unallocated space is a writable area that does not belong to any partitions. Before writing files to this area, you must create a new partition and format it with a file system. Besides creating a new partition, unallocated space can be added to another partition to increase its free space. This is especially helpful when the system C: drive is running full. By adding unallocated space to the C drive, you can resolve this issue without wasting a long time backing up, recreating partitions, and restoring data. This article introduces how to merge and add unallocated space in Windows Server 2016 using both native Disk Management and safe partition software.
1. Add Unallocated Space to Partition With Server 2016 Disk Management
Windows Server 2016 features built-in Shrink Volume and Extend Volume functions within the Disk Management console. "Shrink Volume" is used to decrease a partition's size and create unallocated space, while "Extend Volume" is designed to increase partition size by merging unallocated space. However, many users encounter problems when attempting to add unallocated space in Windows Server 2016. A typical example is that they cannot add unallocated space to the C drive after shrinking the D drive or other partitions.
As you can see in the screenshot, the Extend Volume option is greyed out for both the C: and E: drives after shrinking D.
Why you cannot add unallocated space to the C drive in Server 2016 Disk Management:
- "Extend Volume" can only merge unallocated space into the contiguous partition immediately on its left.
- "Shrink Volume" can only generate unallocated space on the right side when decreasing a partition.
The unallocated space shrunk from the D drive is non-adjacent to C, and the E drive is located on its right side. Therefore, Server 2016 Disk Management cannot add this unallocated space to either the C or E drive.
Additional limitations of Disk Management when combining unallocated space:
- Both the Shrink Volume and Extend Volume functions only support NTFS and ReFS partitions. Disk Management cannot add unallocated space to FAT32 or any other unsupported partition types.
- On an MBR disk, if the contiguous partition D is a Logical Drive, you still cannot extend the C drive even after deleting D.
In short: Disk Management cannot extend the E: drive using the unallocated space on its left. To extend the C drive, you must completely delete D: to obtain contiguous unallocated space. Alternatively, third-party software can move the partition and add unallocated space to any NTFS or FAT32 partition on the same disk.
2. How to Add Unallocated Space to C Drive in Windows Server 2016
Many programs can move and add unallocated space to the C drive in Windows Server 2016. However, it is highly recommended to back up your server first and choose a reliable utility to avoid potential system or partition damage. Unlike other tools, NIUBI Partition Editor features advanced technologies to safeguard your system and data, such as:
- Virtual Mode - All operations are listed as pending for preview, and physical disk partitions will not be modified until you click "Apply" to confirm.
- Cancel-at-will - If you apply incorrect operations, you can cancel the ongoing tasks safely without causing any data loss or partition damage.
- 1-Second Rollback - If any known error is detected while resizing partitions, it automatically reverts the server to its original state instantly. This ensures your server returns online quickly without harmful modifications.
How to Move and Add Unallocated Space to the C Drive on Windows Server 2016:
- Download NIUBI Partition Editor, right-click drive D and select "Resize/Move Volume". Drag the middle of the partition toward the right in the pop-up window to move the unallocated space next to the C drive.
- Right-click the C: drive and select "Resize/Move Volume" again, then drag the right border toward the right to combine the unallocated space.
- Click Apply on the top left to execute. (Disk partitions will not be modified until you click Apply to confirm.)
Pending operations marked with 
can be completed directly within Windows, while those marked with a refresh symbol require a server reboot.
Watch the video how to operate:
The steps remain identical whether you use a physical server with an SSD, HDD, or any hardware RAID array, or a virtual server running in VMware/Hyper-V.
3. Move/Add Unallocated Space to C Drive From Another Separate Disk
On some servers, there may be no other partitions or insufficient free space available on the same disk. Consequently, many administrators wonder if it is possible to move unallocated space to the C drive from another separate physical disk. The answer is no.
No software can transfer space between two separate physical disks because the physical size of a disk drive is fixed. When no free space is available on a disk, you have two options:
- For a physical server: Follow the steps to clone the disk to a larger one and expand the partition(s) using the additional disk space.
- For a virtual server: Follow the steps to increase the size of the virtual disk within VMware or Hyper-V. After doing so, the additional space will appear as unallocated at the very end of the disk. You can then follow the video tutorial above to move and add the unallocated space to the C drive or other partitions.
In Summary
To move and add unallocated space to the C drive in Windows Server 2016, native Disk Management is ineffective in most scenarios. To accomplish this task safely, NIUBI Partition Editor is a much better choice. Besides shrinking, moving, and extending partitions, this utility helps you copy, convert, defragment, wipe, or hide partitions, scan for bad sectors, and perform much more.
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