What is the DCO?
The DCO is the Device Configuration Overlay, which can prevent the whole of the hard drive from being seen. For example, an 120 GB hard drive can be forced to show as a 100 GB hard drive. This allows manufactures to sell that same size hard drives, at different sizes.
The DCO works at a very low level operation, and software forensics tools cannot see past this (at the moment), i.e they cannot image the area protected by the DCO. However, certain cloners like the ICS Solo3 can.
A detailed article on the DCO is available here.