How create new partition on an unallocated space using diskpart?

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 EaseUS Partition Master (Free Edition), or visit the Best Free Partition Management Software page to have a look at other tools.

In case you still prefer to do it the long way, using diskpart, follow these steps :

1. Open the Command Prompt as Administrator. Type diskpart and hit Enter.
2. Once you enter to the diskpart screen type list disk and hit Enter.
3. Now a list of the disks will be shown, type select disk x (the X is the disk number that has an unallocated space) and hit Enter.
4. Now, type create partition primary size=x (The X is the number that you got from the Free column on the previous step) and hit Enter.
5. After the creating is done, type select partition 2 and hit Enter.
6. Now that you are in the new partition, you need to format it by typing format fs=ntfs quick and hit Enter.
7. After that, type assign and hit Enter.
8. Then, type active and hit Enter.
9. Now, your new partition is ready to use.

P.S: Make sure that you select the correct disk that you want to create a new partition on it. Also, Disk 0 is the disk that has the current system installed on.

This is an example of how the above process works: Suppose that I have an 8GB flash drive that it has one partition with size 4GB, and an unallocated space of 3535MB (about 3.5GB). So I need to use the unallocated space and create a new partition to be used.