A field firmware upgrading method is adapted in a field firmware upgrading system having a computing device and a storage device. First, the computing device is boot up to a real-time operating system (RTOS), wherein the RTOS and a firmware of the storage device are communicated with each other via a driver. The RTOS sends reading commands to the firmware, such that the firmware performs a reading operation meeting a prefix proceeding, wherein the prefix proceeding defines an order which specific logic block addresses (LBAs) are read. After the firmware performs the reading operation meeting the prefix proceeding, the RTOS sends writing commands to the firmware, such that the firmware performs a writing operation meeting a specific writing proceeding to write an firmware image file into the storage device beginning at a target LBA defined by the specific writing proceeding.

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