When you save data on a disk, the os places it in an available storage area, or?

When you save data on a disk, the OS places it in an available storage area, or a "Sector".
Usually the computer places that data in sectors in an order and usually next to each other but that is not the case always, sometimes part/s of that data can be scattered at different places which actually slows down the computer, for which we need to Defragment the disk space.   

Bis the