This depends what you mean. The microprocessor controls some aspects of it like the file system, the disk formatting, etc. There is usually a chip on the motherboard or a card responsible for handling data in and out to the hard-drive. There is a software "driver" that helps the PC communicate with the Hard Drive. Once at the hard-drive, there is a microcontroller chip and supporting hardware that controls the decoding and reformatting of data for the drive, the selection of sectors to write to, controlling the motors, and rotation speed and various other things. Finally there are circuits that control the stepper motors for the writing arm, and the placement of the magnetic heads on the platter, then there is circuitry that controls the electromagnetic pulses that go to the head to write information to the drive. So there are lots of components that control how data is written. You can find out more at How Stuff Works.