1) Ensure your source system is not running.
2) First you have to clone the storage. At the command line, run:
VBoxManage clonehd src.vdi dst.vdi
where "src.vdi" is the name of your source disk and "dst.vdi" is the name of your resulting clone disk.
3) Next, open VirtualBox's control GUI and take a look at the properties for your source system (look in the "Details" tab)
4) Click on "New" and create the new clone. When you get to "Virtual Hard Disk", select the "Use existing hard disk" radio button and click on the little folder icon to the right. This will bring up the "Virtual Media Manager".
5) The new cloned drive will (probably) not be in the initial display. Click on "Add" and select the new "vdi" file that you created in step #2 above.
6) Finish creating the new guest.
Sources:
- http://forums.virtualbox.org/viewtopic.php?p=853&sid=1bba6024db1a6e6d1a01bfbff606da83#p858