Chapter 1 What’s New for FortiOS 5.0 : New features in FortiOS 5.0 Patch 6 : Logging Options for 3000 and 5000 Series Models
  
Logging Options for 3000 and 5000 Series Models
To increase stability of the 3000 series and 5000 series FortiGate models, the following changes have been made:
Disk logging is disabled by default, but can be enabled through the CLI.
When disk logging is disabled, it will not appear disk logging as an option on these models.
The default logging method for these models is now memory logging, which has been reintroduced as an option.