def _is_datadisk_exists(existing_disks, datadisk_name):
    """Helper to test if a datadisks exists among
    existing disks.

    :param existing_disks: A list that contains the existing 
                           disks within a storage account.
    :param datadisk_name: The name of the disk to test. You do not need to
                          suffix the name with .vhd.
    :return: True if the disk has been found, False either.
    :rtype: Boolean
    """
    return True if (datadisk_name + ".vhd") in existing_disks else False