Fortigate 3810A TFTP Error – Open boot device failed

You may meet “Open boot device failed” if you are upload a firmware via TFTP. I think Fortigate 3810A doesnt support Fresh firmware install via TFTP for FortiOS 5.0

The workaround is to upload latest 4.x release via TFTP then upgrade to 5.0 via GUI.
As the time I write this post latest release for 5.x is GA patch 6 and for 4.x is MR3Path15.