The hard disk itself is a small computer. Its job is to store a large amount of data and to deliver it to the main computer. Just as a computer a hard disk has a processor, a memory and an operating system (Microcode). A part of the Microcode is on the controller, like the BIOS in the PC. The second part of the Microcode is filed in an exclusive area of the magnetic disks which is inaccessible for the user and operating systems. The rest of the hard disk is the data area. It can be formatted accordingly (Fat, NTFS, Ext2 usw.) and is used to store the data. While booting the computer, an initialization process of the hard disk is done which is comparable with the boot process of the computer. After a short selftest the controller gives the order to start the engine. Then the controller loads its operating system (Microcode) in the memory and signalizes the main computer his readiness for the data exchange. The BIOS of the main computer first asks for the technical data of the hard disk to make sure that the hard disk functions. Once the data are correctly transmitted, the hard disk is recognised by the BIOS.