If you are starting to use powershell, the sheet you can download from the following link is priceless. Print it and keep it on your desk:-) http://www.microsoft.com/downloads/details.aspx?FamilyId=DF8ED469-9007-401C-85E7-46649A32D0E0  

from command line: VBoxManage.exe clonehd source.vhd destination.vhd easy as 1-2-3 :-)