Find the SD card device

In this case, the SD card is /dev/disk4. DO NOT get this wrong or you may destroy all the data on the wrong disk/card/drive.

diskutil list

/dev/disk0
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *160.0 GB   disk0
   1:                        EFI                         209.7 MB   disk0s1
   2:                  Apple_HFS Macintosh HD            159.2 GB   disk0s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk0s3
/dev/disk1
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *160.0 GB   disk1
   1:                        EFI                         209.7 MB   disk1s1
   2:                  Apple_HFS 160GB                   159.7 GB   disk1s2
/dev/disk2
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *320.1 GB   disk2
   1:                        EFI                         209.7 MB   disk2s1
   2:          Apple_CoreStorage                         319.6 GB   disk2s2
   3:                 Apple_Boot Boot OS X               134.2 MB   disk2s3
/dev/disk3
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:                  Apple_HFS Backup                 *319.3 GB   disk3
/dev/disk4
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     FDisk_partition_scheme                        *3.9 GB     disk4
   1:             Windows_FAT_32                         98.6 MB    disk4s1
   2:                      Linux                         1.9 GB     disk4s2

Umount the disk, NOT eject

diskutil unmountDisk /dev/disk4

Write the image to the SD card with dd

dd if=/path/to/image.img of=/dev/rdisk4 bs=1m