Create a Rapsberry PI Rescue Dongle

I encountered multiple issues trying to repartition my SD on my PI. Because the / directory is mounted, it never really worked safely for me to use fdisk. Morevoer some of the powerfull tools such as gparted need X11 installed, which I don’t have by default.

Hopefully the new PI3 B and B+ are able to boot from USB, hence the idea of creating a Rescue Dongle

Consideration regarding USB boot.

It seems that all new PI 3B+ have OTP for USB boot mode setup by default. For the PC 3B, you have to activate using the /boot/config.txt

master-pi is a 3B+, nas-pi and home-pi are 3B:

Let’s check the /boot/config.txt

ansible picluster -i inventory/ -m shell -a "grep program_usb_boot_mode /boot/config.txt" | FAILED | rc=1 >> | SUCCESS | rc=0 >>
program_usb_boot_mode=1 | FAILED | rc=1 >>

The flag for OTP USB flag is set on the 3B+ (by default on master-pi) and on the 3B where I did add the entry to the config.txt (nas-pi)

ansible picluster -i inventory/ -m shell -a "vcgencmd otp_dump | grep 17:" | SUCCESS | rc=0 >>
17:3020000a | SUCCESS | rc=0 >>
17:3020000a | SUCCESS | rc=0 >>

Creation of the Rescue Dongle

  • Flash Raspbian on the Dongle using the normal procedure (Wind32DiskImager,..)
  • Remove the SD card from the PI3.
  • Plug keyboard, mouse, screen … onto the PI
  • Boot the PI3 on the Dongle by removing the SD card
  • Take the time to setup the Raspbian:
    • Enable VNC
    • Enable SSH
    • Setup default hostname
    • Setup password for Pi account
    • Setup resolution (for VNC later)
    • Install tools such as gparted
  • Shutdown the PI and put back the normal SD card.


The idea is to boot a PI from the Dongle and then apply the fixes to the SD card.

  • Shutdown the PI.
  • Remove the SD card.
  • Insert the Dongle into USB port of PI
  • Reboot the PI.
  • Connect to the PI using the VNC or SSH. The PI has started from the OS installed on the Dongle.
  • Insert the SD card into the PI. You will most likely have a popup and the filesystem from the SD card is automatically mounted.
  • Start to use the tools to fix your SD card. For instance:
  • Shutdown and reboot. The PI will restart from the SD card.

Application: Change SD card partition

First step is to reboot a PI without SD card from the Dongle and connect VNC Viewer to it.

First insert the SD and close the popups

In a terminal run, sudo gparted

sudo gparted

unmount the mmc root partition

Resize the current partition (down to 16G), create an extended one and 7*2G logical partition in that 14G partition)

Apply the changes

Shutdown or Reboot. The PI should restart from the SD anyway.

  • TBD