Most drives don't let you read the firmware back. This is for both technical reasons (all the way back to the Seagate 2GB Barracuda, the drive split the firmware and part was in flash and part was on the disk - the flash was just enough to spin the disk up, do a seek and read of the main firmware - the drive knew how to split a firmware image [.LOD file] into the flash and disk parts of it, but the firmware didn't implement reading it back) and for marketing reasons (the companies like HP, Oracle, etc. that sold drives with huge price mark-ups due to being "certified" don't want you converting generic drives).
For example, the Dell firmware images will only apply using the Dell Nautilus tool - they're not in industry-standard format. And the tool checks to see if it is a Dell drive. Even if you override that check, the drive will probably reject the firmware as incompatible. On a "classic" WD drive (not one actually built by HGST) the OEM info is encoded into the part after the dash - like WD2003FYYS-02W0B0. The drive will reject any firmware not matching 02. The rest (W0B0) describes manufacturing variants which may also have different and incompatible firmware - for example some drives changed from 5-platter to 4-platter during production. It also describes mechanical changes unrelated to firmware, for example whether the drive has FDB bearings or not.