How do the hard disk work After replacing it?

It spins for one ( at usually 7200 rpm) and writes in a language that involves 1 and 0's(Binary Code) It is a like a rewritable CD but a lot cooler

A hard disk stores data in much the same way as a floppy drive, magnetic pulses are imprinted onto the surface of the disk by a read/write head which can also then read the data back. Hard disks differ from floppies in that the data can be stored in much higher densities and accessed at much higher speeds by using a non-removable disk which is sealed from contaminants. Modern hard disks usually have several disks within them (platters) and dedicated read/write heads for both sides of each. The platters themselves are usually made of some aluminum alloy coated with a thin layer of iron oxide and are very rigid.

A hard disk drive (HDD), commonly referred to as a hard drive, hard disk, or fixed disk drive, is a non-volatile storage device which stores digitally encoded data on rapidly rotating platters with magnetic surfaces. Strictly speaking, "drive" refers to a device distinct from its medium, such as a tape drive and its tape, or a floppy disk drive and its floppy disk. Early HDDs had removable media; however, an HDD today is typically a sealed unit (except for a filtered vent hole to equalize air pressure) with fixed media.