Know and understand magnetic drives including magnetic hard disks, magnetic tape

3. Storage Devices and Media

Objective

Know and understand magnetic drives, including magnetic hard disks and magnetic tape.

1. Introduction to Magnetic Storage

Magnetic storage uses the principle that a magnetic material can retain a polarity (north or south) which represents binary data (0 or 1). The two most common magnetic storage devices in ICT are:

• Magnetic Hard Disk (HDD)
• Magnetic Tape

2. Magnetic Hard Disk (HDD)

A magnetic hard disk is a non‑volatile storage device that uses rotating platters coated with a magnetic material. Data is written to and read from the surface by a moving read/write head.

2.1 Main Components

• Platters – circular disks coated with a magnetic layer.
• Spindle Motor – rotates the platters at a constant speed (e.g., 5400 rpm, 7200 rpm, 10 000 rpm).
• Read/Write Heads – tiny electromagnets positioned on an actuator arm.
• Actuator Arm – moves the heads radially to access different tracks.
• Controller Circuit – manages data transfer between the computer and the disk.

2.2 How Data Is Stored

Each platter is divided into concentric circles called tracks. Tracks are further divided into sectors (typically 512 bytes or 4096 bytes). The read/write head changes the magnetic polarity of tiny regions called bits to represent 0s and 1s.

2.3 Performance Factors

• Rotational Speed – higher rpm reduces average latency.
• Seek Time – time for the actuator to move the head to the correct track.
• Data Transfer Rate – amount of data moved per second, often expressed as: $$R = \frac{\text{Number of bits per track} \times \text{Rotational speed (rev/s)}}{\text{Time per revolution}}$$

2.4 Advantages and Disadvantages

Advantages	Disadvantages
High capacity (several TB)	Mechanical parts prone to wear and shock damage
Random access – fast retrieval of any file	Higher power consumption than solid‑state drives
Relatively low cost per gigabyte	Performance degrades with fragmentation

3. Magnetic Tape

Magnetic tape is a linear storage medium consisting of a thin strip of plastic coated with a magnetic material. It is primarily used for backup, archival, and large‑scale data transfer.

3.1 Main Components

• Tape Cartridge or Reel – houses the magnetic tape.
• Read/Write Heads – positioned on a rotating drum (in helical‑scan systems) or stationary for linear recording.
• Transport Mechanism – moves the tape past the heads at a controlled speed.
• Controller – manages data encoding, error correction, and communication with the host computer.

3.2 How Data Is Stored

Data is recorded in longitudinal tracks along the length of the tape. Modern tapes use helical‑scan technology where the tape wraps around a rotating drum, allowing high‑density recording.

3.3 Performance Characteristics

• Sequential Access – data is read/written in order; random access is slow.
• Capacity – can exceed 30 TB per cartridge (compressed).
• Data Transfer Rate – typically 100 MB/s to 300 MB/s for current LTO (Linear Tape‑Open) generations.

3.4 Advantages and Disadvantages

Advantages	Disadvantages
Very high capacity at low cost per GB	Sequential access – slower retrieval of individual files
Long shelf life (up to 30 years when stored properly)	Physical handling required; risk of tape wear or breakage
Ideal for offline backup and archival	Requires dedicated tape drive hardware

4. Comparison of Magnetic Hard Disk and Magnetic Tape

Feature	Magnetic Hard Disk (HDD)	Magnetic Tape
Access Type	Random access	Sequential access
Typical Capacity (2025)	0.5 TB – 20 TB	5 TB – 30 TB (compressed)
Average Transfer Rate	150 MB/s – 250 MB/s	100 MB/s – 300 MB/s
Typical Use	Primary storage for operating systems & applications	Backup, archival, bulk data transfer
Reliability	Susceptible to mechanical failure	High durability when stored correctly
Power Consumption	5 W – 10 W (idle)	Very low; only when drive is active

5. Summary

1. Magnetic hard disks store data on rotating platters and provide fast random access, making them suitable for everyday computing tasks.
2. Magnetic tape stores data linearly on a long strip of magnetic material; it excels in high‑capacity, low‑cost, long‑term storage but is slower for random access.
3. Understanding the strengths and limitations of each device helps learners choose the appropriate medium for specific ICT applications such as primary storage, backup, and archival.