The user under which MarkLogic Server is running must have permissions to read the specified directory. For example, on Windows systems, MarkLogic Server typically runs as the SYSTEM user, and therefore the SYSTEM user must have permissions to read the directory (even a mapped network drive directory). Similarly, on UNIX systems, MarkLogic Server typically runs as the daemon user, and that user needs to have read permission to the specified directory. If the user in which MarkLogic Server runs does not have the needed permissions, then an error such as SVC-DIROPEN or SVC-FILOPEN is thrown when you attempt to call this built-in.