By Bryce / Last Updated May 12, 2023

Cannot extend C drive Windows 10

When your C drive is almost full after Windows update or installed with many applications, you may want to extend it to fix the issue. But when you try to increase the size of your C drive, you may find that the option of Extend Volume… greyed out and you are unable to extend C drive in Windows 10 when there is (without) an unallocated space.

Unable to Extend C Drive Windows 10

Solution 1 - Create a qualified unallocated space in Windows 10

When the unallocated space is not right contiguous to the C drive, you will be not able to extend C drive in Windows 10 directly. To make Extend Volume… work, it is necessary to create an unallocated space right contiguous to your C drive. The steps are listed below

Step 1. Right-click D drive (the partition between C drive and the unallocated space) and choose Delete Volume…

* If you don’t want to lose the data stored on the D drive, make a backup before deleting it.

Delete Volume

Step 2. It pops up a window prompting you that deleting this volume will erase all data on it… Click Yes to continue.


Step 3. Now, you’ll get an unallocated space right contiguous to your C drive. Right-click the C drive and choose Extend Volume... and do as the next prompt to complete the operation.

Extend C Drive Windows 10

Solution 2 – Merge non-adjacent unallocated space to C drive in Windows 10 or 11

If you are unable to extend C drive in Windows 10 Disk Management without deleting any partition, you can turn to AOMEI Partition Assistant Professional, which can work under Windows 11/10/8/7, XP, and Vista. It is featured with a function – Merge Partitions, allowing you to add the non-contiguous unallocated space to C drive directly instead of deleting any partition. And if you need, you can resize BitLocker partition to adjust partition size.

Download Demo Win 11/10/8.1/8/7/XP
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The step-by-step guide on how to extend C drive with non-contiguous unallocated space.

Step 1. Install and run it. In the main interface, right-click C drive and choose Advanced Merge Partitions.

Merge Partition

Step 2. In the Merge Partitions interface, choose Unallocated and C:; then, click OK.

Add Unallocated Space To C Drive

Step 3. Here you can preview the operation. Click Apply and Proceed after confirming it.

Apply Merge

Note: AOMEI Partition Assistant can also help you to reduce the size of C drive when you are unable to shrink C drive within Disk Management.

Solution 3 – Allocate free space from D drive to C drive in Windows 10 or 11

When there is no unallocated space on the disk and D drive has enough free space, you can allocate free space from D drive to C drive to extend C drive via AOMEI Partition Assistant Professional. Do the following steps:

Step 1. Install AOMEI Partition Assistant Professional and run it. Right-click D drive and choose Allocate Free Space.

Allocate Free Space

Step 2. In this window, set the amount of free space that you want to allocate from D drive and select C drive as the target drive.

Allocate From D To C

Step 3. You can preview the result after allocating free space to C drive. After confirming, click Apply> Proceed to carry out the operation.

Apply Allocate Free Space


▪ This operation may change the original location for your data but will not delete them, yet, in order to guarantee the safety of your data, you can learn the tutorial "How to Safely Partition" before executing this operation.
▪ To extend C drive without losing data in Windows Server 2019/2016/2012/2008/2003, you can use AOMEI Partition Assistant Server.

Wrap things up

You will be unable to extend C drive in Windows 10/11 Disk Management when there is no contiguous unallocated space. It is acceptable to get a proper unallocated space by deleting partition. But the whole process is time-consuming. Thus, it is advised to choose one from the latter two solutions. If there is unallocated or free space, pick solution 2. If there is no such space, you can choose solution 3.

If the entire system disk is too small, you can copy the disk to a larger disk via AOMEI Partition Assistant, thus you can replace it with a larger one without reinstalling Windows and other applications. If you don't want to change partiton size of C drive when it is almost full, you can transfer installed apps on C drive to another drive.