Resize partition of Windows 7 running on VirtualBox with dynamically allocated storage

I run Windows 7 inside VirtualBox.

I resized the disk of Windows 7 from 25 GB to 50 GB:

VBoxManage modifyhd Windows\ 7\ Pro.vdi --resize 50000
0%...10%...20%...30%...40%...50%...60%...70%...80%...90%...100%

PROBLEM: I can' grow the partition, neither with Windows 7 itself nor with GParted:

It looks like VirtualBox does not tell the client OS about the new size.
What additional step is necessary?

Answer

You can only resize disks if they are not part of a VM that uses snapshots.

If you are using snapshots, you can easily clone the state to a new VM and hard disk, and resize that one. VirtualBox 4 now supports cloning from the GUI, so it's super easy.

VBoxManage - modifyhd

Here's a helpful walk through of the whole process, Resize and Expand a Virtualbox Hard Drive and Media in 4 Steps