Device configuration overlay (DCO) is a hidden area on many of today's hard disk drives (HDDs).
Usually when information is stored in either the DCO or host protected area (HPA), it is not accessible by the BIOS (or UEFI), OS, or the user.
This permanently alters the disk, unlike with the host protected area (HPA), which can be temporarily removed for a power cycle.
LinEn 6.01 was validated by the National Institute of Justice (NIJ) in October 2008, and they found that "The tool does not remove either Host Protected Areas (HPAs) or DCOs.
Their findings indicated that "If a physical acquisition is made of a drive with hidden sectors in either a Host Protected Area or a Device Configuration Overlay, the tool does not remove either an HPA or a DCO.