I wouldn't suggest using diskpart if the hard drive already has some data on it, especially if you've installed the system on it. This is because if you don't know how to use it, then it's better to avoid using it to prevent you from data loss.You can use a third party tool to do that instead, which will give you the same results.Use (Free Edition), or visit page to have a look at other tools.In case you still prefer to do it the long way, using diskpart, follow these steps:. Open the Command Prompt as Administrator.

Method 2: Add Unallocated Space to System Partition C on Windows 10 for Free You also have another choice to merge unallocated space to C drive on Windows 10 with Diskpart command. Diskpart is a command interpreter, which allows you to mange hard disk on Windows system such as resizing hard disk, assigning a drive letter to disk partition and deleting partition, etc.