Hard disk and Optical disk

Hard disk noun - A rigid magnetic disk mounted permanently in a drive unit.

Optical disk and hard disk are semantically related In some cases you can use "Optical disk" instead a noun phrase "Hard disk".

Optical disk noun - A disk coated with plastic that can store digital data as tiny pits etched in the surface; is read with a laser that scans the surface.

Hard disk and optical disk are semantically related in computer storage topic. You can use "Hard disk" instead a noun phrase "Optical disk".