FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
Article Id 193503
Sometimes a FortiGate disk may need to be formatted before the disk can be used.

This is particularly important for HA clusters:
1) when forming an HA cluster, disk status must match
2) logging of event logs to disk is helpful to understand HA cluster transitions

Otherwise, care should be taken to use disk logging selectively and only when needed.
Consider using alternate log locations like FortiAnalyzer, FortiCloud and syslog.

Expectations, Requirements
FortiGate should be able to log to disk if it has one.

1. At the CLI run:
get system status

Look for ...
Log hard disk: Need to Format 

2. Format the disk 
execute formatlogdisk

!! This operation will erase all quarantine files and logging data on the hard disk and will reboot the FortiGate