Cannot merge partitions of external hard drive
You can not NTFS format from macOS. You may have been using the ExFAT format.

Below are some possible commands to create a single formatted partition.

For MS-DOS FAT32 format use:

diskutil  partitiondisk  disk2  1  mbr  FAT32  EXTDISK  R


For ExFAT format use:

diskutil  partitiondisk  disk2  1  mbr  ExFAT  EXTDISK  R


For Mac OS Extended (Journaled)

diskutil  partitiondisk  disk2  1  mbr  JHFS+  EXTDISK  R


For example, if you want the ExFAT format, the below is the output you should see by entering the appropriate commands. Of course, your 2 TB drive will probably be sightly different in size from mine.

Davids-iMac:~ davidanderson$ diskutil list disk2
/dev/disk2 (external, physical:
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     FDisk_partition_scheme                        +2.0 TB     disk2
   1:                 DOS_FAT_32 EXTDISK                 43.5 MB    disk2s1
   2:               Windows_NTFS extdisk                 1.8 GB     disk2s2
Davids-iMac:~ davidanderson$ diskutil partitiondisk disk2 1 mbr ExFAT ExtDisk R 
Started partitioning on disk2
Unmounting disk
Creating the partition map
Waiting for partitions to activate
Formatting disk2s1 as ExFAT with name ExtDisk
Volume name      : ExtDisk
Partition offset : 2 sectors (1024 bytes)
Volume size      : 3906250061 sectors (2000000031232 bytes)
Bytes per sector : 512
Bytes per cluster: 131072
FAT offset       : 2048 sectors (1048576 bytes)
# FAT sectors    : 120832
Number of FATs   : 1
Cluster offset   : 122880 sectors (62914560 bytes)
# Clusters       : 15258309
Volume Serial #  : 5a7b6783
Bitmap start     : 2
Bitmap file size : 1907289
Upcase start     : 17
Upcase file size : 5836
Root start       : 18
Mounting disk
Finished partitioning on disk2
/dev/disk2 (external, physical:
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     FDisk_partition_scheme                        +2.0 TB     disk2
   1:               Windows_NTFS ExtDisk                 2.0 TB     disk2s1
Davids-iMac:~ davidanderson$