Here are the steps to backup the SD card:
1. Shutdown the Raspberry Pi:

sudo shutdown -h now

Remove the SD card and insert it into your laptop’s card reader.
2. On your laptop’s local terminal program type:

mkdir ~/Desktop/Pi_BU

to make a folder for the backup image
3. Then type:

df -h

and look for your SD card. Make note of the disk number. It should look something like: /dev/disk3s1
4. Unmount the SD card:

diskutil unmount /dev/disk3s1

(change the 3 to your card’s disk number)
5. Start the backup:

sudo dd if=/dev/rdisk3 of=~/Desktop//pi_BU/wheezy-backup.img bs=1m

Wait about 10 minutes for an 8GB card