How to boot up Raspberry Pi 3 from external hard disk

The original source of this page can be found at: https://www.maketecheasier.com/boot-up-raspberry-pi-3-external-hard-disk/.  I copied it here for my future reference and to correct a couple of errors that I found in the tutorial.  The original page is a better resource than this one as it contains better formatting and illustrations.  If you find yourself on this page, please refer to the original.

The requirements for this tutorial:

  1.  A Raspberry Pi 3
  2. A microSD card (minimum 4 GB) with PIXEL installed.  (This tutorial assumes that you already have a working PIXEL installed on your microSD card.
  3. An external hard disk formatted to Ext4.

Setting Up External Hard Disk

  1.  Insert the microSD card into the Raspberry Pi 3.  Plug in the external hard drive to the USB port of  the Raspberry Pi 3.  Power up the Pi.
  2. Once you have reached the desktop, open a terminal.  Log into the root account and mount the external hard drive.
    sudo su
    mount /dev/sda1 /mnt
  3. Next, we need to install Rsync (if it is not already installed):
    apt-get install rsync
  4. Copy all of the files from the microSD card to the external hard drive.  We are using <code>rsync</code>, so all the file permissions and ownership are intact.
    sudo rsync -axv / /mnt
  5. With all the boot up files in the external hard drive, we need to modify the startup file so that it is pointing to the external hard disk for boot up instructions.
    cp /mboot/cmdline.txt /boot/cmdline.txt.bak
    nano /boot/cmdline.txt
    We need to edit two parts of this line. Change the root= to /dev/sda1, and at the end, add rootdelay=5.
  6. Lastly, we are adding the hard drive entry to “/mnt/etc/fstab” so the root folder in the external hard drive is automatically mounted during boot up.
    nano /mnt/etc/fstab
    Add this line to the second line of the file:
    /dev/sda1 / ext4 defaults,noatime 0 1
    Add a # at the start of the last line to disable booting up from the microSD card:
    #/dev/mcblk0p7 / ext4 defaults,noatime 0 1
    Note: /dev/mncblk0p7 is referring to your microSD card slot and the value might differ in your case.
    After the changes it should look like this:
    proc /proc proc defaults 0 0
    /dev/sda1 / ext4 defaults,noatime 0 1
    /dev/mncblk0p6 /boot vfat defaults 0 2
    #/dev/mncblk0p7 / ext4 defaults, noatime 0 1
    That’s it. Reboot your Pi, and it should boot up and run from the external hard drive. One thing to note is that the microSD card needs to be in the slot, as the Pi needs to read the startup file from it before it boots up from the external hard drive.

Optional: Increase the swapfile size

Assuming your external hard drive comes with tons of space, you might want to increase the swapfile size so your Pi can run slightly faster.

  1.  Open a terminal and log into the root account.
    sudo su
  2. Edit the swapfile.
    nano /etc/dphys-swapfile
    Change the value of CONF_SWAPSIZE from 100 to 512.
  3. Restart the service to update the changes.
    sudo dphys-swapfile setup
    sudo /etc/init.d/dphys-swapfile stop
    sudo /etc/init.d/dphys-swapfile start

Conclusion

The Raspberry Pi 3 comes with several useful improvements such as higher RAM, a WiFi module and a power supply big enough to support an external hard drive.  This makes it useful to run bigger and more intensive projects.  As such, the microSD card with a small storage size can be a limiting factor, not to mention its slow read/write speed and it being susceptible to data corruption.  With the instructions above, you can now power on your Raspberry Pi from the external hard drive and improve its performance.