function ExtractFileDrive(const FileName: string): String;
Returns a string containing the 'drive' portion of a fully qualified path name for the file passed in the FileName. For file names with drive letters, the resulting string is in the form '<drive>:'. For file names with a UNC path the resulting string is in the form '\\<servername>\<sharename>'. If the given path contains neither style of path prefix, the result is an empty string.