For what' s it' s worth, I would only temporary enable disk logging on those little guys if I really needed to troubleshoot something.
From personal experience the flash-based " hard drives" in those small-to-mid-side units are acceptable to various failures -- of the 80CM and 200Bs we have deployed (over 50 units) in the last 4-6 years we had to RMA (guessing) 10-15 units (mostly 200Bs) due to " bad" flash.
Personally, Fortient disabling disk logging on those smaller units was the " right thing" to do. But if you want to " re-enable" disk logging, you have to make sure the firmware supports a " hard drive" . On later firmwares, Fortinet removed " hard drive" functionally from some models. You can check from the CLI by typing:
get hardware status
NSE4/FMG-VM64/FortiAnalyzer-VM/6.0 (FWF30E/FW92D/FGT200D/FGT101E/FGT81E)/ FAP220B/221C