Basically a hard disk consists of a stack of aluminium, alloy, or glass platters 5.25, 3.5 inch or even smaller than this in diameter.
On each platter is deposited on thin magnetizable metal oxide.
There is no any information whatsoever on the disk after manufacturing it.
Each platter of the disk must receive a low-level format done by the software before the disk can be used.
The format consists of a series of concentric track where each tracks contains some number of sectors having short gaps between the sectors.
The figure given below shows the format of a sector:
The last level in designing/developing the disk for use is to perform high-level format of each partition.
This operation of high-level format lays down a boot block, the free storage administration, root directory, and an empty file system.
The high-level format operation also puts a code in the partition table entry telling which file system is used in the partition because many OSs multiple incompatible file systems. Therefore at this point the computer system can be booted.
Now, when the power of the computer system is turned on, then the BIOS runs initially and then reads in the master boot record and jumps to it.
This boot program then checks to see that which partition is active.
Now after checking the partition for its activeness, it reads in the boot sector from that partition and runs it.
Basically the boot sector of a computer system contains a small code that searches the root directory for a specific program. That certain program is then loaded into the memory and executed.