In a traditional hard disk drive (HDD), the rotating disk platters are coated on both sides with which material to store data?

Introduction / Context:
This question explores how traditional hard disk drives store data. Inside an HDD, one or more platters spin at high speed while read write heads move just above their surfaces. The way data is stored on these platters depends on a special coating that can be magnetised in tiny regions. Understanding this coating helps you see why hard disks are classified as magnetic storage devices and distinguishes them from optical or solid state alternatives.

Given Data / Assumptions:

• The focus is on hard disk drives used as secondary storage in computers.
• The question asks what material coats the platters on both sides.
• Options mention carbon, magnetic metallic oxide, optical metallic oxide, plain aluminium and silicon.
• We assume a conventional HDD design with metallic platters and magnetic surfaces.

Concept / Approach:
Hard disk platters are usually made from aluminium or glass substrates. To store data, these substrates are coated with a very thin layer of magnetic metallic oxide or other magnetic alloy. The read write heads magnetise small regions of this coating in different directions to represent bits of data. A thin carbon layer may also be added on top for protection, but the actual data storage happens in the magnetic layer. Optical metallic oxides are used on CDs and DVDs read by lasers, and silicon is used in solid state chips, not on HDD platters. Therefore, the correct answer is that hard disk platters are coated on both sides with magnetic metallic oxide.

Step-by-Step Solution:

Verification / Alternative check:
Hardware references and manufacturing descriptions of HDDs consistently state that platters are coated with magnetic materials such as cobalt based alloys or magnetic oxides. These materials allow precise magnetisation and demagnetisation for reliable data storage. They are applied as thin films on both sides of the platter to maximise capacity. Additional layers, such as carbon overcoats and lubricants, protect the magnetic layer but do not change the fact that data is stored magnetically. This confirms that the key functional coating is magnetic metallic oxide and not purely carbon, optical coatings or semiconductor silicon.

Why Other Options Are Wrong:

• A thin carbon protective layer only: Carbon overcoats exist but serve as protection, not as the main data storing layer.
• Optical metallic oxide used for lasers: Used on optical discs like CDs and DVDs, not on magnetic hard disk platters.
• Plain aluminium without any coating: Aluminium can form the substrate, but without magnetic coating it cannot store magnetic data.
• Silicon semiconductor material: This is used in chips and SSDs, not as the coating on HDD platters.

Common Pitfalls:
Students sometimes mix up details of optical and magnetic storage, especially when both involve thin coatings on discs. A simple rule is that if a laser is used to read the disc, the technology is optical, and if magnetic heads are used, the technology is magnetic. Hard disks use read write heads and rotating platters in a sealed enclosure, so the data storing layer must be magnetic. Remembering that a hard disk is a magnetic device helps you quickly select magnetic metallic oxide as the correct coating when this question appears in exams.

Final Answer:
In a hard disk drive, the platters are coated on both sides with a Magnetic metallic oxide coating to store data.