No way without downtime. You need to enter the BIOS during reboot, using the serial connection. Press any key when prompted, then select to format the disk.
Afterwards, the firmware image, signature databases and configuration will be gone!!
So prepare this operation and have the firmware image and config file ready.
The most convenient way to reload these, given a non-functional FGT without IP address, is to copy them to a USB stick and reboot the FGT with the stick attached. The boot process will compare firmware version and config and restore one or both if necessary.
The files on the stick need to have these filenames:
for me, it concerns only the log disk, which has never been formated, so I think that the process described by ede_pfau is not necessary.
To format log disk, run the following command: 'exec formatlogdisk', and of course it will reboot the device. Currently, you have configured the Fortigate to log on disk, but the disk is not ready to log.
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.