Most modern computers store the data and programs they are not immediately using on disks: magnetic disks, called hard disks or hard drives for those that are usually built in, or floppy disks, also known as diskettes, or just floppies, for those that are regularly inserted and removed; and optical disks, often called CDs (Compact Disks): CD-ROMs (Read Only Media), CD-Rs (Recordable), and CD-RWs (Read-Write). The device that accesses a disk for a computer is called a disk drive; a hard disk is usually built in to its hard drive, so those terms are often used interchangeably. This category contains programs for working with these disks: testing, checking, scanning, repairing errors, formatting, defragmenting, using different data storage formats, such as for different operating systems, and similar utilities.