Hard disks are mechanical forms of computer memory consisting of spinning platters of magnetic materials inside a sealed unit. Magnetization is used to store compartmentalized information in binary code, by turning the magnetized piece of the drive "on" or "off." A single hard disk will contain billions of these individual points of information (bits) aggregating to larger amounts of storage, such as bytes or megabytes. Hard disks are vulnerable to magnetic fields.
- Hard disk from britannica.com