Create a disk image from an SD card and write the disk image to another SD card (Mac OS X)

SDImage.sh

#!/bin/bash

### NOTE
# Ideally, you should create an image from small partition (e.g., 4 GB) instead of the whole SD card (e.g., 32 GB).
# For example, an image for Raspbian image should be created by the following procdure:
#   (1) Install the official Raspbian image (3.5  GB for Jessie) on an SD card
#   (2) Manually expand the partition to a modest size to accommodate your base software (e.g., 4 GB)
#   (3) Perform apt-get update and upgrade, install software and configuration that you want.
#   (4) Create an image from that (4 GB) partition
#
# The instruction for resizing partion and creating disk image with a specific size can be found in this link:
#   http://elinux.org/RPi_Resize_Flash_Partitions


## Read SD card to disk image
# STEP 1: insert the source SD card
diskutil list		

# STEP 2: From the output, find the device number corresponding to your SD card 
# 				E.g., /dev/disk2
#         In the code below, replace "diskX" below with your disk number

# STEP 3: copying (and compressing)
sudo dd if=/dev/rdiskX of=~/Desktop/Chatberrypi.dmg bs=1m

# OPTIONAL: 
#    You can experiment with different block sizes (The "bs" above; "1m" 
#    means 1 MB.) Run dd for a few seconds and press Ctrl + C to see 
#    the transfer rate. Adjust "bs" and test again.
#    
#    In my MacBook Air, I found 1m yield an acceptable performance

# STEP 3 (alternative): compress the image on-the-fly. (Note: Don't check the progress of dd with the command at the end of this page)
sudo dd if=/dev/rdiskX conv=sync,noerror bs=1m | gzip -c > ~/Desktop/Chatberrypi.img.gz

# STEP 4: remove the source SD card



## Write disk image to SD card

# STEP 5: insert the destination SD card
diskutil list		

# STEP 6: replace "diskX" below with the device number of the SD card (see STEP 2)

# STEP 7: unmount the SD card
diskutil unmountDisk /dev/diskX

# STEP 8: format the SD card
sudo newfs_msdos -F 16 /dev/diskX

# STEP 9: write the disk image
sudo dd if=~/Desktop/Chatberrypi.dmg of=/dev/rdiskX bs=1m

# STEP 9 (alternative): decompress the image on-the-fly and write image. (Note: Don't check the progress of dd with the command at the end of this page)
sudo gunzip -c ~/Desktop/Chatberrypi.img.gz | sudo dd of=/dev/rdiskX conv=sync,noerror bs=1m


# OPTIONAL:
# 	To check progress of dd, open a separate terminal window and run
# 	the following command to ask dd to print progress (in its terminal) 
# 	every 20 seconds.
while sudo killall -INFO dd; do sleep 20; done


# Sources:
# SD card back up: https://smittytone.wordpress.com/2013/09/06/back-up-a-raspberry-pi-sd-card-using-a-mac/
# Status checking: http://www.commandlinefu.com/commands/view/5440/check-the-status-of-dd-in-progress-os-x