This article gives easy solution if you cannot/unable to extend system C drive in Windows 10 Disk Management. Drag and drop to shrink, move and extend partition.
When system C: drive is full in Windows 10, many people want to extend it, thinking this problem can be solved without reinstalling Operating System and programs. Yes, it is possible but you are unable to extend C drive with Windows 10 built-in Disk Management utility.
Two reasons why unable to extend C drive
Reason 1: there must be Unallocated space next to C drive.
As we know, a 256GB hard disk can't be increased to 512GB, it is physical unit and the space must be transferred inside it. Thus, before extending a volume, there must be Unallocated space. As the name, it is not allocated to any partition.
To get Unallocated space via Windows 10 Disk Management, you may either delete or shrink a volume. If you delete a volume, all its disk space will be converted to Unallocated but you'll lose all files in it. Generally speaking, the adjacent drive D is used to install all kinds of programs, so you cannot delete it.
Obviously, it is much better to get Unallocated space by shrinking a volume, because you won't lose any data and all settings keep the same. But, the Shrink Volume function only give you option to enter an amount, then Unallocated space will be generated on the right side for you.
After shrinking volume D, Extend Volume is disabled for all other partitions. Because, this function can only expand the partition when there is adjacent Unallocated space on the right side of it.
C and all its left partitions are non adjacent to the Unallocated space, E is on the right side, so Extend Volume is unavailable.
If Shrink Volume can make Unallocated space on the left side, or Disk Management can move partition, there is no such issue.
Reason 2: there is limit between Primary and Logical partition
If you use GPT disk or there is no Logical partition in the disk, you may ignore this paragraph.
On the bottom left of Disk Management window, you'll see if there is Logical partition.
To check your disk type: Press Windows and X on the keyboard and select Disk Management, right click Disk 0 (or other) and select Properties. Switch to Volumes tab in the pop-up window.
Shrink Volume cannot get adjacent Unallocated space by shrinking D, some people tried to delete it. It works if D is a Primary partition, but if it is Logical, you still cannot extend C drive.
After I delete the Logical drive D, Extend Volume is still disabled for C drive. Have you noticed the difference: disk space of D is showing Free instead of Unallocated?
If I right click this Free space, it only allow me to create a new volume, I'm also unable to extend C drive, so I continue to delete another logical drive E.
Now when I right click the Free space, there is an option to delete partition. After deleting it, Extend Volume is enabled for C drive.
Free space deleted from a Logical partition cannot be extended to any Primary partition. Unallocated space deleted from a Primary partition cannot be extended to any Logical partition. You must delete all Logical partitions in the Extended partition, then the Free space will be converted to Unallocated.
Obviously, you can't extend C drive by deleting all other Logical partitions.
Solution of unable to extend partition C
If you have shrank drive D via Windows 10 Disk Management, you need 3rd party software such as NIUBI Partition Editor to move Unallocated space to the left side.
Step 1: Right click drive D and select "Resize/Move Volume", drag middle position rightwards in the pop-up window.
Step 2: Right click drive C and select "Resize/Move Volume" again, drag right border towards right to combine Unallocated space.
If you deleted volume D but cannot extend C, because original D is Logical, simply run NIUBI and follow Step 2 to combine the adjacent Unallocated space.
Unable to extend volume C to another disk
Neither native Disk Management nor third party software is unable to expand C drive with space in another disk. If there are no other volumes in the same disk, you may copy this disk to a larger one with NIUBI Partition Editor, then extend C drive with extra disk space.