A Hard disk drive is a form of computer memory consisting of spinning magnetic platters inside a sealed case. Magnetization is used to store compartmentalized information in binary code, by spinning the hard disks (called platters) of the drive past read/write heads, which set small points (which represent bits) to a north or south polarity. A single hard disk will contain billions of these individual points of information, which are aggregated to form larger incraments of storage, such as bytes or megabytes. Hard disks are vulnerable to magnetic fields, although they are encased in metal, so the platters themselves are somewhat protected.
- Hard disk from britannica.com