When I had this error, some articles suggested that you need to install the upgrade the boot loader first before attempting the firmware upgrade. That the Boot loader (.rfb file) can be found on the same zip folder as the firmware. This does not work in this case.
SG300 Firmware Upgrade
What is a Firmware? A firmware is software that is embedded on the router. This firmware is normally updated to include new features and enhancements to the device. All of our firmware upgrades are FREE.
Solution: The steps below fixed the issue(SG300 Firmware Upgrade).
– All that needs to be done to correct this issue is to perform an interim firmware upgrade to the previous version. Then upgrade form this version to the next, till it gets to the most recent one.
The switch flash file system was changed in 1.3.7.18