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1. Open Terminal and go to /dev

$ cd /dev
2. Now it’s time to find your flash drive in /dev. Typically it’s /dev/diskN where N could be 0, 1, 2, 3, etc

$ diskutil list
/dev/disk0
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *121.3 GB   disk0
   1:                        EFI                         209.7 MB   disk0s1
   2:                  Apple_HFS Macintosh HD            120.5 GB   disk0s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk0s3
/dev/disk3
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *8.1 GB     disk3
   1:                        EFI                         209.7 MB   disk3s1
   2:       Microsoft Basic Data KB                      7.9 GB     disk3s2
OK! It’s disk number 3, i.e. /dev/disk3 Remember the name of partition (it’s KB in my case). Your flash drive is mounted as /Volumes/NAME

3. Let’s erase and reformat the disk. In 90% you will get a message that the system can’t unmount your drive.

$ diskutil eraseDisk MS-DOS KB /dev/disk3
Started erase on disk3
Unmounting disk
Error: -69888: Couldn't unmount disk
Use the partition name from step 2.

$ diskutil unmountDisk force /Volumes/KB

$ diskutil eraseDisk MS-DOS KB /dev/disk3