increase client win7 disk size on Linux

The original disk space was 20 GiB, so windows keep asking me that they need a space.  I found the web site http://jonmifsud.com/blog/increase-virtualbox-disk-size/. And follow what he did, and got the 50 GiB disk space.

~/.VirtualBox/HardDisks$ VBoxManage modifyhd
Usage:

VBoxManage modifyhd         <uuid>|<filename>
                            [--type normal|writethrough|immutable|shareable|
                                    readonly|multiattach]
                            [--autoreset on|off]
                            [--compact]
                            [--resize <megabytes>|--resizebyte <bytes>]
~/.VirtualBox/HardDisks$ ls
Debian7.vdi  win7_data.vdi  win7.vdi

~/.VirtualBox/HardDisks$ VBoxManage modifyhd  win7.vdi --resize 51200
0%...10%...20%...30%...40%...50%...60%...70%...80%...90%...100%

~/.VirtualBox/HardDisks$

After restarting Win 7, 

Computer -> [Right mouse] management (G) -> Disk management  -> Right mouse on  [C:]  (one can see the empty space after [C:] ->  Volume expand (X) -> And following their explanation. 

See what I got. (Korean Windows)