Why you need to merge C drive with unallocated space?
Your hard drive may not divided well at the first time. You can even see that there
is still an unallocated space on the hard drive when you open the Windows Disk
Management. Meanwhile, the C drive is running out of space. Under this
circumstances, merge C drive with unallocated space should be the best way to extend system partition.
Using Windows Disk Management to merge c drive with unallocated space
Windows Disk Management has been improved since Windows 7. You can use it to extend
volume or shrink volume according to your requirements. If the unallocated space is
at the right side of C drive and adjacent to C drive, you can use it to add
unallocated space to C drive.
Right-click My computer, select Manage, and open the Disk Management.
Then, right click the C drive, click Extend Volume.

Then, you can get into the extend volume wizard and merge C drive with unallocated
space.
However, the Windows Disk Management supports you merge C drive with unallocated
space only when the unallcated space adjacent to C drive and it must be at the right
side of C drive. Other wise, you cannot extend volume to unallocated space because the Extend Volume would be
grayed out.

Windows Disk Management Alternative
As Windows Disk Management cannot merge C drive with unallocated space which isn’t
adjacent to C drive, or at the left side of C drive. Here we recommend you the best
Windows Disk Management alternative AOMEI Partition Assistant, which can help you add unallocated space to C drive no
matter where the unallocated space is. To merge C drive with unallocated space, here
you can follow the steps below:
Step1. Install and launch AOMEI Partition Assistant, you will
see the main environment of your hard drive. Right click the C drive at the main
interface, then, select Merge Partitions at the pop out menu.

Step2. The, it will move to the window where you can add the
unallocated space to C drive by selecting the partitions in the box. Click OK.

Step3.Here you can see that you have already extend system partition
by adding the unallocated space to C drive. Don’t forget to click Apply to submit the whole operations.

Extend C drive with unallocated space should be the best way to enlarge system drive.
You do not need to change a hard drive, or reingstalling windows, but add the
unallocated space to C drive. Besides, AOMEI Partition Assistant can adjust your
hard drive to the best status with Allocate
Free Space if there is no unallocated space on your hard drive.