The only way this is possible currently is to use the syslogng package. There you can set a location to use for log storage which can be a different drive or partition. If you then set /var as a ram drive system logging does not write to the drive.

BUT.... there is no built in management of different drives in pfSense so you need to use custom scripts to do that. Hence it's not supportable.

pfSense is not configured to support multiple disks other than hardware raid or geom mirrors (or ZFS pools). There is no provision to mount additional disks in the GUI. It has been done before but some custom scripting is required to make sure services relying on additional disks are handled gracefully should the disk not be mounted for example.

For logging you can install the syslog-ng package and specify the storage location on another disk. Then export the pfSense logs to it. Again though you can edit the fstab to mount that but it may be overwritten.

Hosting a syslog server externally is the recommended way to go.

Steve