Cloning a Raspberry Pi SD card

The goal is to clone the SD card of a Raspberry Pi after setup and installation, perhaps for backup or to make more copies of the same setup. Copying the SD card as an image file (.img) onto a regular machine is simple - just install Win32DiskImager (Windows) or Etcher (Windows/Mac) and click through the prompt. The tricky part is writing the .img file back into other SD cards.

All SD cards are not made equal. Not for cards that claim to have the same capacity. Not even for cards of the same stated capacity that come from the same Amazon order. Off by one byte and Win32DiskImager / Etcher would complain about the card being too small to hold the image. The simple solution is to use a bigger card. Card sizes grow by a factor of two, so you'd need a 32GB card to hold the image of a 16GB card (pretty much the smallest size you can find in Q4 2017). It works, but how'd you backup your 32GB card?

Another way to get around this is to resize the .img file. Usually very little space is actually used on a newly prepared Pi (a 2017 Raspbian Lite image with RabbitMQ, pika, numpy, etc. all installed for both Python 2 and 3 takes only 1.6GB of space), so most of the card is actually empty and can be trimmed from the image file. The process goes roughly as follow: boot a machine using a GParted / Ubuntu Live image, mount the storage device (e.g. USB stick) holding the .img file, mount the .img file as a loopback device, resize the file system using GParted, unmount the loopback device, then truncate the .img file.

First boot into the live image (if you are using the Ubuntu Live image, make sure you don't accidentally install it). You will also need to mess with the boot order of your machine at least temporarily. There's extensive resource elsewhere for this. Once in, mount the USB drive holding the .img file (if it isn't automatically mounted; use lsblk to find out what the device name is). The partition on my USB drive shows up as /dev/sdb1. My .img file is named node-080.img):

sudo mkdir /media/usb0
sudo mount /dev/sdb1 /media/usb0

Now mount the .img file as loopback device:

sudo losetup -f (take note of its output; mine is /dev/loop1)
sudo losetup -P /dev/loop1 /media/usb0/node-080.img

Launch GParted:

sudo gparted /dev/loop1

Resize the second partition (the first one is the /boot partition, which is tiny) to the content, leaving ~200MB of empty space at the end. Commit and exit.

Unmount the loopback device:

sudo -d /dev/loop1

Now truncate it, but first find out where the last sector of the partition is:

sudo fdisk -lu /media/usb0/node-080.img

Take note the sector size (512 bytes for me) and End sector. Then truncate:

truncate --size=$[(END_SECTOR+1)*512] /media/usb0/node-080.img

Verify:

ls -al