I've just had the exact same situation last month. When I received the FGT, I tried to install a HQIP image - it did not even load. Very likely a disk failure. I have not heard anything other than this from the RMA department yet.
@ehammons , @ede_pfau as Alex mentioned, when the FortiGate fails to recognize its own inbuilt disk, especially during booting/when trying to format from BIOS, there isn't anything that can be done, really, and it becomes a case for the RMA team.
+++ Divide by Cucumber Error. Please Reinstall Universe and Reboot +++
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