Yes, windows is in a dirty state. Google “windows dirty bit” for more details on this issue. The issue isn’t with FOG but Windows and how it closes the files on the disk.
To fix this you will need to restart the workstation and then:
Use the proper sysprep command switch to power off the computer (best) Sysprep the system then use shutdown.exe -s -t 0 to power off the computer.Since Windows 8 windows start button shutdown isn’t really a shutdown, but an enhanced sleep state (S5). Using one of the above methods will ensure the system is properly closed and ready for imaging.