dmitryk

Microcontroler in a sence is specialized CPU - Processor, which sits on the HDD PCB - back side of the drive, where all components are located.
On modern drives it is the largest chip. Microcontroler runs firmware - code which stored in the flash or mask ROM on the same PCB. That is where all drive functionality is done. Complexity of the modern drive is so sgnificant that it takes milions and milions lines of code to run to allow you access data on your drive.