Magnetic disks are the most popular direct-access secondary storage device. They are also the most popular on-line secondary storage device.
A magnetic disk is a thin, circular plate/platter of metal or plastic. Its surfaces on both sides have a coating (such as iron oxide) that can record data by magnetization. Data is recorded on its coated surfaces as tiny, invisible magnetized, for recording data. The disk itself is stored in a specially designed protective envelope, or cartridge, or several of them are stacked together in a sealed, contamination-free container.
Like in the case of magnetic tapes, we can erase old data and record new data on magnetic disks as well. As we record new data on it, it erase old data in the same area automatically. However, we can read data many times without affecting it.