If most of the adaptive information is on the service zone of a hdd...why does the
MCU chip still have to be transferred over? What critical info is stored on this chip?

thanks...so why do HDD manufactuers keep on changing the firmware so often? surely, standardization would be cheaper for them?

Because of the ever changing specifications of the hardware.

for instance every flash drive that may look exactly the same, same PCB, same controller chip, but different brand NAND chip has to have a different firmware file written to it. slightly different timing for the electronics, slightly different communication requirements etc.

also bug fixing (parallel to the windows updates I guess)