Adding new partition/hdd
Just follow these steps on how to add another new partition/virtual hdd into your existing Guest OS
Start your Virtual Box, and click on File->Virtual Media Manager
On the Virtual Media Manager, click on New button.
Just click next as usual, setup your preferred harddisk size.
One it's done, it will listed down in the available virtual hard drive list, and click OK.
Now back to your Virtual Box, click your Guest OS, and click Settings button.
Now go to Hard Disks, click the add button..make sure it's primary slave, and click OK after you've done it.
Now start your OS...then one it loads, right-click on My Computer->Manage.
On the Unallocated partition, right-click on it and select New Partition.
Click next, and select your hdd.
Untick when it asks if you want to convert it as dynamic..then just click next.
Once it's done, it will be displayed in your My Computer.
Should have reserved few posts back earlier @_@