further googling makes it appear you have to use reset in conjunction with the console, so will try that
found this for 60d, does it hold true for 90d?
Connect to the FortiGate 60D using a console cable.
Plug the FortiGate 60D to the power adapter and wait for the device to boot up.
FortiGate-60D (15:09-08.12.2013) Ver:04000022 Serial number: FGT60D1111111111 CPU(00): 800MHz Total RAM: 2GB Initializing boot device... Initializing MAC... nplite#0 Please wait for OS to boot, or press any key to display configuration menu......
Got so much exposure in resetting 50E's (around a hundred times) since we distributed 400 of these to bank branches.
The $1 trick is:
1. Unplug the power.
2. Watch and wait for the LEDs to start to blink (LAN, WAN and STA leds).
3. After about 5 seconds after the STA led blinks, press the small reset button at the back using a paper clip, keeping it pressed until you see the leds blink orange, then red. At this stage, you can release the button.
4. Congratulations. You have manually factory-reset your Fortigate.
The Fortinet Security Fabric brings together the concepts of convergence and consolidation to provide comprehensive cybersecurity protection for all users, devices, and applications and across all network edges.