If you happen to be using a FortiGate and you don’t see logging to disk as an option when in the GUI you may need to enable it via CLI. You can do this by entering the following commands in CLI:

Config Log Disk Setting
Set Status Enable
End

After you have done this, log out of the GUI and log back in. You should, from this point on see disk logging in the log access section of the FortiGate. If it is for some reason STILL not showing up you may need to diagnose the hard disk to see if it has possibly experienced a hardware failure. You can do this by utilizing the following command at CLI to determine status etc:

diag hardware deviceinfo ide hda

It will provide you with information on the disk to determine what is up.