Fixed disk and Hard drive

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

Hard drive and fixed disk are semantically related in hard disc topic. In some cases you can use "Hard drive" instead a noun phrase "Fixed disk", when it comes to topics like hard disk.

Hard drive noun - Computer hardware that holds and spins a magnetic or optical disk and reads and writes information on it.

Fixed disk and hard drive are semantically related in hard disc topic. You can use "Fixed disk" instead a noun phrase "Hard drive", if it concerns topics such as hard disk.