Using the command line interface (CLI) : execute : execute format disk
 
execute format disk
The execute format disk command allows you to format the hard disk on the FortiDB system. Executing this command will erase all device settings/images, VPN & Update Manager databases, and log data on the FortiDB system's hard drive. FortiDB's IP address and routing information are preserved.
Syntax
execute format disk
When you run this command, FortiDB prompts you to confirm the request.
Warning: If you use this command without executing backup all settings command, you may not be able to view assessments or reports after you archive and restore your data. When you want to archive and format disk, make sure that you execute config system backup all-settings command before archiving.