Reset and reformat external SSD to APFS I have got this 256GB external SSD that I had divided in two partitions with 2 bootable versions of macOS: 10.11 in one partition and 10.12 in another.

Now I want to just have one APFS partition, but restoring the disk from Disk Utility did not help. I cannot get to a single APFS partition.

This is the output of diskutil list:

  /dev/disk0 (internal, physical):
     #:                       TYPE NAME                    SIZE       IDENTIFIER
     0:      GUID_partition_scheme                        *121.3 GB   disk0
     1:                        EFI EFI                     209.7 MB   disk0s1
     2:          Apple_CoreStorage Macintosh HD            121.0 GB   disk0s2
     3:                 Apple_Boot Boot OS X               134.2 MB   disk0s3

  /dev/disk1 (internal, physical):
     #:                       TYPE NAME                    SIZE       IDENTIFIER
     0:      GUID_partition_scheme                        *1.0 TB     disk1
     1:                        EFI EFI                     209.7 MB   disk1s1
     2:          Apple_CoreStorage Macintosh HD            999.3 GB   disk1s2
     3:                 Apple_Boot Recovery HD             650.0 MB   disk1s3

  /dev/disk2 (internal, virtual):
     #:                       TYPE NAME                    SIZE       IDENTIFIER
     0:                  Apple_HFS Macintosh HD           +1.1 TB     disk2
                                   Logical Volume on disk0s2, disk1s2
                                   5642582C-EEF0-4BFF-AE7E-D2E4BA96E592
                                   Unencrypted Fusion Drive

  /dev/disk3 (external, physical):
     #:                       TYPE NAME                    SIZE       IDENTIFIER
     0:      GUID_partition_scheme                        *250.1 GB   disk3
     1:                        EFI EFI                     209.7 MB   disk3s1
     2:                 Apple_APFS Container disk4         124.8 GB   disk3s2
     3:                 Apple_Boot Recovery HD             124.3 GB   disk3s3

  /dev/disk4 (synthesized):
     #:                       TYPE NAME                    SIZE       IDENTIFIER
     0:      APFS Container Scheme -                      +124.8 GB   disk4
                                   Physical Store disk3s2
     1:                APFS Volume SSD Storage             860.2 KB   disk4s1
     2:                APFS Volume Senza titolo            864.3 KB   disk4s2


I see the two partitions as separate disks in Disk Utility:
Open Terminal and get the details

diskutil list


Below I assume the external "Apple_Boot/Recovery HD" has the disk identifier disk3s3 and the APFS container disk3s2.
Remove "Recovery HD".

diskutil eraseVolume "Free Space" %noformat% /dev/disk3s3

Expand "Container disk4" to fill the free space.

diskutil apfs resizeContainer disk3s2 0

Delete volume (here the volume "Senza titolo")

diskutil apfs deleteVolume disk4s2