While I was trying to add a virtual disk to a headless virtual machine, upon running

VBoxManage modifyvm MachineName --hda DiskName.vdi

this error occurs:

ERROR: Could not find a storage controller named 'IDE Controller'

This can be resolved by replacing

<StorageControllers /> 

with

<StorageControllers>
  <StorageController name="IDE Controller" type="PIIX4" PortCount="2">
  </StorageController>
</StorageControllers>

Actually, scratch everything above. Use the following tutorial: http://forums.virtualbox.org/viewtopic.php?f=7&t=28273