How to extend disk space in windows 2012
Login to windows. Click on Start run and type cmd.
In windows command prompt.
run diskpart.exe
DISKPART prompt will come.
diskpart >list volume
Above command will show list of volumes.
Select volume volume number
It will select the volume which you want to extend.
extend filesystem
It will extend filesystem size.
Once done. Close the command prompt and refresh windows explorer. If still it is showing old space then try rebooting the server.