: Connect to the FortiGate using a serial console cable.
: Performing a factory reset on certain high-end models (MBD/FIM) can accidentally remove the image file, leading to boot failures that require manual re-imaging as detailed in this Fortinet Community Technical Tip . open boot device failed fortigate
Follow the prompts to set the server IP and local IP, then download the image. : Connect to the FortiGate using a serial console cable