Extend Windows 7 VirtualBox Disk

1. Use the VirtualBox client to create a new disk of the size you want in the same directory as your guest.
2. Clone the old disk to the new.  Open a command prompt, navigate to the guest directory and run :

VBoxManage clonehd   --existing

3. Attach the new disk to the guest and detach the old disk.  Ensure the new disk is first in the list / SATA 0
4. Guest should now boot up.  If you find it asks you to select a boot disk then the order is incorrect as per step 3
5. Open Windows disk manager, It will currently show the old size with #GB free.  Right click, extend, follow the prompts.
6. Confirm everything is good and you can delete the old disk.

Thats it, you're done ...