function ExtractFileDrive(const FileName: String): String;
Returns a string containing the 'drive' portion of a fully qualified path name for the file passed in 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.