What's Hard Drives ?
Hard Drives are magnetic memory devices of computer.
Their size is 1.8in(Mini Laptop computer, Music Player), 2.5in(Laptop PC), 3.5in(Desktop PC).
Consists of outer case(Aluminum), motor, disks, magnets, etc.
Magnet is neodymium magnet, very strong.
Neodymium is a kind of rare earth element.