Always have a console cable connected during the upgrade so you can monitor the boot process and catch any errors in real-time.
For the 100D, the 6.2 branch (specifically versions like 6.2.13 or later) is often considered the terminal stable point. It balances modern security features with the hardware's CPU and RAM limits. Fortigate 100d Firmware Download BEST
Use the MD5 or SHA256 checksums provided on the Fortinet support site to verify the integrity of your downloaded file. Always have a console cable connected during the
Before every single step in the upgrade path, download a full configuration backup. Fortigate 100d Firmware Download BEST
Never download FortiOS images from unofficial forums or "free" mirrors. These files can be injected with backdoors that compromise your entire network. Choosing the Best Firmware Version