By Dervish / Last Updated March 7, 2022

What’s unallocated space for?

After a period of time, you may find that your computer runs slowly or cannot react to commands immediately, or even your screen becomes gray so that it cannot work at all. Those problems may be caused by the same reason: your system partition is out of space. This can be proved when you look into Computer, and you will have no surprise to see that the system partition which usually is assigned with drive letter C shows a red bar due to low drive space, while other partitions may show a green bar with enough drive space. In order to solve the problem, you could extend the C drive with unallocated space. But how to create unallocated space when there is none?

C Drive Full

Create unallocated space with Disk Management

Windows 11/10 inbuilt Disk Management is able to create unallocated space and you can achieve this in two different ways - either shrink a partition or delete a partition.

Method 1: Shrink Partition

1. Right-click the partition you want to shrink and click “Shrink Volume”.


2. Type the number of sizes you want to get as unallocated space.

Enter Size

3. Now you get the unallocated space.

Unallocated Space

As you can see, there is an unallocated space created behind I: drive. But, in this situation, you cannot use Disk Management to extend C drive with the unallocated space, because DM asks the unallocated space must be adjacent and behind the partition which is needed to be extended. Therefore, you should use a third-party software to finish the rest job, such as AOMEI Partition Assistant which is able to merge non-adjacent partitions, or you can change to create unallocated space by deleting the partition next behind the C drive.

Method 2: Delete Partition

1. Right-click I: drive, and select “Delete Volume”. (This will delete everything on the partition, so please backup before doing so.)

Delete Volume

2. Now you have created the unallocated space. Then you can use DM to extend C: drive with it.

Extend Volume


Maybe for some of you, deleting the partition will be treated as a risk of losing data. Then, there is an easier and safer way to extend your C drive without unallocated space, that is, allocate free space from one partition to C drive via AOMEI Partition Assistant.

Additional information

AOMEI Partition Assistant Professional is a professional partition manager, it not only provides with resize/move, merge, split, copy, create, delete, format, delete the partition, and all other basic partition features, but also supports some advanced features like making bootable media, converting NTFS and FAT32, create Windows To Go, and of course allocate free space, etc. Next, we show you how to allocate free space from your D: drive to C: drive.

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1. Install and launch AOMEI Partition Assistant Professional Edition.

2. Right-click D drive, and select “Allocate Free Space”.

Allocate Free Space

3. In pop-out window, specify the free space to be allocated and the destination partition that the free space will be added to.

Allocate From D To C

4. Now you can preview the result, and remember to click “Apply”.

Apply Allocate Free Space

What’s your purpose to create unallocated space? If you do want to use an unallocated space to extend partition, then using AOMEI Partition Assistant to allocate free space is the best option as you can know.