WDS Cannot Add Driver

If you were trying to add a driver to a boot wim and it failed resulting in error code the following may help.

• Unmount temporary .wim from Temp location using DISM: dism /unmount-Wim  /MountDir:<mountDir> /discard

or

• Clear all your temp directories.
• Browse to “HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\WIMMount\Mounted Images” and delete any keys below this.

WDS should now allow you to add drivers to the boot.wim.

Source: http://social.technet.microsoft.com/Forums/en-US/w7itproinstall/thread/a202816a-59f6-48bb-a71a-fe2ca3add4c9/