In order to clone an SD Card using MacOSX, first insert the SD Card. Then open the terminal and enter following command in order to find out which device the SD Card is in your system:
df -h
The ouptput will be something like
Filesystem Size Used Avail Capacity iused ifree %iused Mounted on
/dev/disk5s1 128Mi 108Mi 20Mi 85% 512 0 100% /Volumes/NO NAME
Now you can create an image of the device /dev/disk5s1, which is your SD Card using dd command:
sudo dd if=/dev/disk5s1 of=./SDCard.img
If you prefer a compressed image, make use of piping it trough gzip
sudo dd if=/dev/disk5s1 | gzip > SDCard.img.gz
