This is typically caused by another application, such as a virus scanner, accessing the drive while the defragmenter is running. The easiest way to test this is to try running the defragmenter in safe mode. If it is successful there, then you can typically close open applications in normal mode to narrow down which one may be causing this interference. The virus scanner in our application can cause this as well so it needs to closed if it is running. You can do this from the settings in the program.