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