~ $ diskutil list /dev/disk2
/dev/disk2 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *5.0 TB     disk2
   1:                        EFI EFI                     209.7 MB   disk2s1
   2:                  Apple_HFS Timemachine (WD Elem... 750.1 GB   disk2s2
   3:                  Apple_HFS Space (WD Elements -... 4.3 TB     disk2s3
~ $ diskutil eraseVolume free free /dev/disk2s2
Started erase on disk2s2 Timemachine (WD Elements - Fattest)
Unmounting disk
Error: -69879: Couldn't open disk
~ $