I format a Fortigate 80c and I can`t upload the boot firmware

Hi people, I have a Fortigate 80c and today I must  reboot it, after that never boot again. I get an error and can't start it.

So after a couples of tests I decided to flash it.

Start the firewall, press F to format the boot device, then G connect to TFTP and upload the boot image, after upload and uncompresing, press D to make it my default boot image and after a few seconds I get a error like "You must format the boot device"

I stuck and I need to fix this issue ASAP.

Any ideas?

thanks