Fix Can't Extend C Drive with Unallocated Space in Windows

Updated by

Joey Albert 

Written by

Shelly 

Updated on May 27, 2026

Does your C drive get full for no reason? It's easy to find "Low Disk Space" error pops up on your system partition after you use Windows for a certain period. Then, you'll start facing problems, such as slowed-down performance, constant low-storage errors, etc.

To get rid of this issue, as an experienced Windows user, you may use Disk Management to easily increase the C drive space with unallocated space left behind your system partition. However, some Windows users report that Windows doesn't allow them to do so because the unallocated space isn't left on the C drive.

On this page, I'll take extending the C drive in Windows 11 as an example to show you the complete process of enlarging the system C drive without losing any data, time, or computer performance.

Why Can't Extend C Drive with Unallocated Space?

What exactly is the real cause that blocks you from extending the C drive even with unallocated space? Here's a checklist for you to learn:

1. There is a partition between the C drive and the unallocated space
2. The unallocated space is not on the right side of C
3. The unallocated space is on a different disk, not the system disk

Now, you can run a quick diagnostic to see if you are in one of the above-listed states. If so, you'll get an "Extend volume greyed out" error when trying to run Disk Management to extend the C volume.

So, how to fix this error? Dig into the next part, and you'll find efficient methods to get your C drive enlarged successfully.

How to Fix "Can't Extend C Drive with Unallocated Space" Error? 

Now, you've clearly noticed which state you are in. If your C drive is not adjacent to the unallocated space, or the unallocated space is on a different disk, follow the lead here to get the right solution.

Don't forget to share this guide online to help more of your friends. Ready to get started increasing your C drive space? Let's dive into the respective guide now.

Method 1. Use Disk Management to Delete a Neighbor Volume to Extend C Drive

To make space on a physical drive, it must feature unallocated space. Since we don't have anyone on our hard drive, we'll be taking some from the Neighbor partition.

You can delete the neighbor partition to extend the C Drive with the following steps:

Step 1. Open Disk Management by right-clicking the Windows icon and selecting "Disk Management".

Step 2. Locate and right-click on the neighbor partition behind the C drive and select "Delete Volume".

Step 3. Click "OK" to confirm.

Once the space is unallocated, continue with the next step.

Step 4. Right-click on the C drive and select "Extend Volume...".

Step 5. Expand your C drive with the whole available unallocated space by clicking "Next", "Next" and "Finish".

Method 2. Use Partition Software to Extend C Drive with Non-adjacent Unallocated Space 

Note that there's no way to add non-adjacent unallocated space to your system partition without the help of a third-party tool. EaseUS Partition Master, with its reliable resize/move and data migration features, is exactly the tool you need for help.

Option 1. Directly Extend C with Non-adjacent Unallocated Space

Step 1. Shrink a neighbor partition to extend partition.

If the hard drive has enough unallocated space to extend the target partition, jump to Step 2, and continue. If not, free up space first:

• Go to the Partition Manager section, right-click on a partition next to the target partition and select "Resize/Move".
• Drag the partition ends to free up space, leaving enough unallocated space behind the target partition, click "OK".

Step 2. Extend the target partition with unallocated space.

Right-click on the target partition you want to extend and select "Resize/Move".

Drag the target partition handle into the unallocated space to extend partition.

Step 3. Keep all changes.

Click the "Execute Task" button and click "Apply" to keep all changes.

Option 2. Extend C with Unallocated Space Across Disks

When the unallocated space exists on another disk, you can apply the Data Migration feature to enlarge the C drive. 

Step 1. Right-click on the D: partition with enough free space and select "Allocate Space".

Step 2. Select the target partition - C: drive and drag the partition panel rightward to add the free space from D: drive to C: drive. Then click "OK" to confirm.

Step 3. Click the "Execute Task" button and click "Apply" to start the process of moving free space from D: drive to C: drive.

Alongside this, EaseUS Partition Master also allows you to merge C and D and move space from D to C, etc., to enlarge the C drive.

Conclusion: 

The "Can't Extend C Drive with Unallocated space" error is fixable. To get rid of this issue, you can either use the Disk Management tool within Windows or EaseUS Partition Master. 

For a Windows beginner or if you prefer a secure method, we'll recommend the latter because it covers up all the ends and ensures a smooth partition extension for a beginner.