A computer storage medium usually in the form of a rigid circular aluminium plate with a metallic magnetic coating on both sides; this form is known as a hard disk. The flexible form, known as a floppy disk, has a magnetic coating of ferric oxide on both sides of a polyester base. Hard disks have a larger storage capacity than floppy disks; the latter provide a low-cost lightweight storage medium. In both forms data is stored in and retrieved from a set of concentric tracks in the magnetic coating. This is achieved by means of a disk drive, in which one or more disks may be rapidly rotated. Read-write heads in the disk drive are moved radially over each disk surface so that data may be written to (i.e. stored in) or read from the required track. Specific storage locations in the track may be accessed directly and in any order. Unlike floppy disks, hard disks are usually permanently mounted within the disk drive. See also moving magnetic surface memory.