Thank you for posting,
I understand you are trying to update the firmware on the Intel® SSD 520 Series.
Please note the Intel® SSD 520 Series already has the latest firmware; therefore, there is no need to update it. You may verify the firmware update is available for other models using the following link (and the first models list):
You mentioned the drives are properly recognized by the motherboard and operating system. Just in order to make sure the drives are working correctly you may run a diagnostic test with the Intel® SSD Toolbox: