A Memory That Survives the Power Cut
It is easy to forget, but every time you switch off a computer you are doing something a little remarkable. The machine goes completely dark. The chips stop. The screen, the fans, the lights — everything that needed power is gone. And yet your photos, your documents, your operating system are all still there, waiting, the next time you turn it on. The part of the computer that pulls off this quiet trick is the drive that stores your files.
Magnets Holding the Story
A traditional hard disk drive holds data on round metal platters coated with a magnetic material. The platter spins, and a tiny arm holding a read/write head hovers just above the surface. To save data, the head magnetizes very small regions of the coating, flipping each one into one of two directions. Those directions stand for the ones and zeros that make up every digital file. Once a region is magnetized, it stays that way on its own. No current is needed to keep it in place. That is the key idea: magnetism is a stable, physical state of the material.
Why That Means It Survives Without Power
Memory that holds its contents without electricity is called non-volatile. A hard drive is non-volatile because the magnetic patterns it lays down do not depend on anything being powered on. When the computer reboots, the head moves back over the platter, reads the orientation of each region, and the data is reconstructed exactly as it was. Compare this with RAM, the computer's short-term memory. RAM is volatile — it needs constant electricity to hold its state, and the moment power is cut, its contents vanish. That is why the desktop you had open disappears on a hard shutdown but your saved files do not.
A New Kind of Drive, Same Idea
Many computers today use solid-state drives instead of spinning platters. An SSD has no moving parts. Inside, it uses flash memory chips that trap electric charge in tiny cells, and those trapped charges stay in place even with the power off. The technology is completely different — silicon instead of magnets — but the underlying property is the same: the cells are non-volatile, so the data stays. Whether magnetic or solid-state, the trick is to use a physical state that the material holds on its own, so the story of your files does not need to be plugged in to keep being told.
Source
This article was written using information from Wikipedia.