a device called a head reads and writes data in a hard drive by manipulating the magnetic medium that composes the surface of an associated disk platter. naturally, a platter has 2 sides and thus 2 surfaces on which data can be manipulated; usually there are 2 heads per platter, one per side. (sometimes the term "side" is substituted for "head," since platters might be separated from their head assemblies, as with the removable media of a "floppy" drive.)