Mac OS

Check which disks are mounted in you computer:

$ df -h
Filesystem      Size   Used  Avail Capacity  iused   ifree %iused  Mounted on
/dev/disk1     112Gi  102Gi  9.1Gi    92% 26932856 2393446   92%   /
devfs          186Ki  186Ki    0Bi   100%      643       0  100%   /dev
map -hosts       0Bi    0Bi    0Bi   100%        0       0  100%   /net
map auto_home    0Bi    0Bi    0Bi   100%        0       0  100%   /home

Insert the SD and run again df -h, check how the new disk is named:

df -h
Filesystem      Size   Used  Avail Capacity  iused   ifree %iused  Mounted on
/dev/disk1     112Gi  102Gi  9.6Gi    92% 26803002 2523300   91%   /
devfs          187Ki  187Ki    0Bi   100%      647       0  100%   /dev
map -hosts       0Bi    0Bi    0Bi   100%        0       0  100%   /net
map auto_home    0Bi    0Bi    0Bi   100%        0       0  100%   /home
/dev/disk2s1    56Mi   20Mi   36Mi    36%      512       0  100%   /Volumes/boot

Notice that the new disk is named disk2 and has a partition disk2s1

Now umount the SD:

umount the disk:
diskutil unmountDisk /dev/disk2

Now flash the image in the SD:

#Flash uncompressed image
sudo dd if=/path/to/image/erle_image.img of=/dev/rdisk2 bs=8m conv=sync,notrunc,noerror

#Flash compressed image
gunzip -c /path/to/image/erle_image.gz | sudo dd of=/dev/rdisk2 bs=8m