def _get_existing_logical_drives(raid_adapter):
    """Collect existing logical drives on the server.

    :param raid_adapter: raid adapter info
    :returns: existing_logical_drives: get logical drive on server
    """
    existing_logical_drives = []
    logical_drives = raid_adapter['Server']['HWConfigurationIrmc']['Adapters'][
        'RAIDAdapter'][0].get('LogicalDrives')
    if logical_drives is not None:
        for drive in logical_drives['LogicalDrive']:
            existing_logical_drives.append(drive['@Number'])
    return existing_logical_drives
