Open Virtualbox OSE, go to File > Virtual Media Manager(VMM) or just Ctrl + D. Click New button on the Hard Disk Tab. Follow the instructions, until finish. Make sure this time take care about the size of the partition. And finish the procedure (choose the right partition type, name and size. Now we have created a new Virtual Hard Disk (VHD).
2. Add the new VHD to the VCO
Exit the VMM by clicking OK button. On the Virtualbox OSE, Right Click the targeted VCO, go to setting.. (Make sure the VCO is Powered Off) Go to Hard Disks. Click the Add Attachment Button (Button with + ) and your newly created VHD will be inserted automatically and finish it with OK button.
3. Start targeted VCO
The VCO will detect and install the new VHD when startup and wait until it finish and the Right Click on My Computer, choose Manage. Computer Management will be opened and go to Storage > Disk Management. Your new VHD will be in the list but labeled as Unknown. At the time you click the Disk Management, there will be a popup for Disk Initialize and go through the procedure until finish. The new VHD will be detected as Unallocated.
4. Format the new VHD
Right click the Unallocated drive and choose New Volume. Go through the Procedure until finish depends on your requirement. And now you already have new disk partition on you VCO.