Error during build

An error occurred while saving the following file:

C:\Program Files\National Instruments\LabVIEW 8.5\vi.lib\Robotics Library\NIFPGAInterface\DIO\WritePWMPeriod.vi

Invoke Node in AB_Source_VI.lvclass:Close_Reference.vi->AB_Build.lvclass:Copy_Files.vi->AB_Application.lvclass:Copy_Files.vi->AB_Build.lvclass:Build.vi->AB_RTEXE.lvclass:Build.vi->AB_Engine_Build.vi->AB_Build_Invoke.vi->AB_Build_Invoke.vi.ProxyCaller
<APPEND>
Method Name: <b>Save:Target Instrument</b>

I get this at the end of my build. What does this mean?