System memory and hard disks
If the FortiGate unit has a hard disk, it is enabled by default to store logs. This also means that you do not have to enable this and configure the settings for logging to the hard disk, but modify these settings so that it is configured for your network logging requirements.
If the FortiGate unit has only flash memory, disk logging is disabled by default, as it is not recommended. Constant rewrites to flash drives can reduce the lifetime and efficiency of the memory. It must be enabled in the CLI under config log disk setting.
For some low-end models, disk logging is unavailable. Check a product’s Feature Matrix for more information. In either case, Fortinet recommends using either a FortiAnalyzer unit or the FortiCloud service.