I want to extend the size of my /root directory but no data should lost during extending
When asking Linux questions, it is usually good to mention the distro that you are using.
Why does it have to /root? With linux it really does not matter where you put a new harddrive. As long as you enter the correct data into fstab you are ok.
Partition which is mounted to / i want to extend the same
You might want to look into this for resizing your partitions: https://gparted.org/livecd.php