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.

  1. Try using Red + Blue button (HAOS 8.5 and later)
  2. Use rpiboot

Reinstall Home Assistant OS using Red + Blue button

  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.

Reinstall Home Assistant OS using rpiboot

  1. Install rpiboot from github.com/raspberrypi/usbbootca
  1. Using Windows Installer here
  2. From source code here
  1. Open Home Assistant Yellow to access jumpers on the PCB
  2. Set JP1 to USB
  3. Set JP2 (USB-C Rcvry)
  4. Plug-in the USB-C cable to Home Assistant Yellow and connect it to your PC
  5. Run rpiboot
  6. Run Raspberry Pi Imager and flash the Home Assistant OS Installer onto the eMMC
  7. Remove power from the board
  8. Remove JP2
  9. Set JP1 to UART
  10. The Home Assistant OS Installer is now flashed to the internal eMMC. On next boot, the installer will install the latest stable version of Home Assistant OS. You can now 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