But many of you may suffer from the low space error on Windows 10.
You canmove unallocated space to C driveto fix this problem easily.
Here are three methods for you to extend the C drive space.

Step 2: SelectCdrive, then selectExtend Partitionon the left pane.
After that, adjust the button to determine how much space you want to add to the C drive.
Step 4: ClickOKto preserve your change.

After all these steps, you have added unallocated space to the C drive successfully.
But the unallocated space should be adjacent to and behind the C drive.
you’re able to work with the following steps if the condition is met.

Step 1: Right-select theWindowsicon and chooseDisk Managementfrom the menu.
Step 2: Right-press theCdrive, then selectExtend Volumefrom the context menu.
Step 4: ClickFinishto confirm the decision and complete the operation.

Disk Management will execute the decision you made on the disk immediately.
Therefore, you should be careful when making changes.
Here, I will introduce you to how to add unallocated space to C drive with this tool.
Step 1: PressWin + Rto pop fire up Run window.
Step 2: Typecmdinto the text box and pressShift + Ctrl + Enterto run Command Prompt as administrator.
Step 3: punch in the following command lines and hitEnterat the end of each.
Final Words
All the methods above can help you add unallocated space to C drive without formatting.
But you should be careful when making changes to the C drive.
Hope this post can give you some useful information.