I tried a small test and it was successful. I then tried it with the following syntax for a large 38 gb directory structure:
robocopy "\\gacnyw2k1\Share_root\Share\Military" "\\gacnyw2k4\data\Military" /COPYALL /S /E
After about two hours it got stuck copying a file and displayed an "ERROR 5, Access is denied", "Waiting 30 seconds ... Retrying". It kept repeating this sequence and never continued so I had to cancel the command. I logged into another computer and checked that file and it was not in use (no users were accessing any files in the entire directory structure). From this other computer, I moved that sub-directory containing that file outside the source structure, deleted the partially copied structure and started the ROBOCOPY command again. It will take hours until I know if it is successful or gets stuck elsewhere.
I was logged in with full administrator rights to all files on both computers so it was not a rights issue and no user was locking the file. If it was really locked, I would not have been able to move that file.
Can anyone explain why this error occurred and what can be done to prevent it from stopping ROBOCOPY again?