10-23-2018, 04:45 AM
It's most likely not available because the partition is active and in use by the operating system. You have to resize it offline with software like gparted or similar. By that I mean you boot the computer from a Linux distribution with disk management tools like gparted and resize the C partition through the tools.
GParted: https://gparted.org/
GParted: https://gparted.org/