With IPHost Network Monitor you can run simple snmp requests against a Fortinet device in your network.
fgSysDiskCapacity
fg Sys Disk Capacity
1.3.6.1.4.1.12356.101.4.1.7
Total hard disk capacity (MB), if disk is present
Back to FORTINET-FORTIGATE-MIB MIB page.
IPHost Network monitor allows you to monitor fgSysDiskCapacity on Fortinet device via the SNMP protocol. Download IPHost Network Monitor (500 monitors for 30 days, 50 monitors free forever) to start monitoring Fortinet access points right now.