Did you already tried to format your external disk using drive manager? There is an option to format the whole drive with NTFS file system?
Or as an alternative you could try to do all steps manually.
- delete partition How to Delete Partition in Linux {Using the fdisk Utility}
- create new partition How to Create Partitions in Linux {Using parted and fdisk Commands}
- format partition with NTFS file system How to Format Disk Partitions in Linux {ext4, NTFS and FAT32}