Step 1 : find your SD card
In the terminal, run
diskutil list and look for the SD device. Mine was
** Step 2: backup your card**
Run the following command:
sudo dd if=/dev/rdisk2 of=backup.img bs=1m
if is the input file,
of the output file, and
bs the block size. This will, however, create a backup the size of your SD card, even if it's not full. My 8GB card created an 8GB image even though only 2GB was used.
NOTE: I used
/dev/rdisk2 instead of
/dev/disk2 because it's about 8 times faster.