Ensure your 100D has no flash memory errors before starting, as the upgrade process writes heavily to the internal storage. Installation Best Practices
Always have a console cable connected during the upgrade so you can monitor the boot process and catch any errors in real-time. Fortigate 100d Firmware Download BEST
Fortinet no longer releases new feature updates for this model. Ensure your 100D has no flash memory errors
Always read the "Resolved Issues" and "Known Issues" sections of the release notes before upgrading. Fortigate 100d Firmware Download BEST