Note: It is very unlikely that this is necessary. Before going through this step, make sure to use the built-in device wipe mechanism.

By default the Raspberry Pi Compute Module 4 has a fixed boot order which tries to boot from the internal eMMC before booting from a USB mass storage device. In rare cases it can be that the internal eMMC is deemed bootable, but does not boot into a working system. In this case, there are two options you can try to reinstall Home Assistant OS.

Option 1 (recommended): Reinstall Home Assistant OS using Red + Blue button

This option works with Home Assistant OS 8.5 and later only.

  1. Remove power from your system
  2. Power up the system
  3. Within 5s (or from start), press and hold the red and blue button on the backside of the device simultaneously.
  4. Hold the two buttons until only the red LED is lit.
  5. Remove power from your system
  6. Follow the instructions to install Home Assistant OS using the Home Assistant OS Installer from a USB flash drive.

Option 2: Reinstall Home Assistant OS using rpiboot

  1. Install rpiboot onto your computer.
    • Using Windows Installer here
    • From source code here
  1. Open Home Assistant Yellow enclosure to access jumpers on the PCB.
  2. Set JP1 to USB.
  3. Bridge JP2 (USB-C Rcvry).
  4. Plug-in a USB-C cable to Home Assistant Yellow and connect it to your PC.
  5. Power up the board. Only the red LED should be on at this point (solid, not blinking).
  6. Run rpiboot on your PC and let it run through. Afterwards, the green LED should be on.
  7. Run Raspberry Pi Imager and flash the Home Assistant OS Installer for Yellow onto the eMMC ("RPi-MSD"). Refer to Step 1 through 5 from the Installing Home Assistant Software on Kit procedure for the exact steps. Windows might bring up an message asking to insert a disk, this message can be closed. The Raspberry Pi Imager prompt to remove the SD card can be ignored as well. Press Continue and close the Raspberry Pi Installer.
  8. Remove power from the board.
  9. Un-bridge JP2.
  10. Set JP1 to UART
  11. Follow the instructions here. Note that because the installer is now installed to the eMMC, no USB flash drive is required.

Resources:

Raspberry Pi Steps to Flash eMMC