Switch back to HFS+ from APFS Recently I heard about new apple file system.
Of course I wanted to try it.
So I moved all files to one external hard drive for safety... And accidentaly switched it to APFS...
How to revert changes?

$ diskutil apfs createContainer /dev/disk1s2
Started APFS operation on disk1s2 Data
Creating a new empty APFS Container
Unmounting Volumes
Switching disk1s2 to APFS
Creating APFS Container
Created new APFS Container disk1s2
Finished APFS operation on disk1s2 Data

$ diskutil list
/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *240.1 GB   disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                  Apple_HFS Macintosh SSD           239.2 GB   disk0s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk0s3

/dev/disk1 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk1
   1:                        EFI EFI                     209.7 MB   disk1s1
   2:                 Apple_APFS                         999.9 GB   disk1s2

/dev/disk2 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *500.1 GB   disk2
   1:                        EFI EFI                     209.7 MB   disk2s1
   2:                  Apple_HFS Untitled                499.8 GB   disk2s2

/dev/disk3 (disk image):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     Apple_partition_scheme                        +25.2 MB    disk3
   1:        Apple_partition_map                         32.3 KB    disk3s1
   2:                  Apple_HFS Data Rescue             25.1 MB    disk3s2
I'm afraid there's no non-destructive way to revert back to HFS once converted. You may use diskutil apfs deleteContainer /dev/disk(x)s(x) (where x is your correct APFS container).
This will revert the APFS container back to HFS but will destroy all data. Best find another pool of storage to copy all data from that drive to, then revert, then copy back.