Every now and then when I build my solution here (with 7 projects in it) I get the dreaded \'Command copy exited with code 4\' error, in Visual Studio 2010 Premium ed.
If you are running Windows 7 onward, you can try the new 'robocopy' command:
robocopy "$(SolutionDir)Solution Items\References\*.dll" "$(TargetDir)"
More information about robocopy can be found here.