With IPHost Network Monitor you can run simple snmp requests against a Fortinet device in your network.
fgSysDiskUsage
fg Sys Disk Usage
1.3.6.1.4.1.12356.101.4.1.6
Current hard disk usage (MB), if disk is present
Back to FORTINET-FORTIGATE-MIB MIB page.
IPHost Network monitor allows you to monitor fgSysDiskUsage on Fortinet device via the SNMP protocol. Download IPHost Network Monitor (500 monitors for 30 days, 50 monitors free forever) to start monitoring Fortinet WAN extenders right now.