In the opening paragraphs, some quick answers to key questions are: No, a fixed hard disk drive (HDD) and a solid state drive (SSD) are not the same. The main differences are:
- HDDs use spinning magnetic disks to store data while SSDs use integrated circuits and have no moving parts.
- SSDs are faster, more durable, and more power efficient than HDDs.
- HDDs have larger capacities and are cheaper per gigabyte than SSDs.
Hard disk drives (HDDs) and solid state drives (SSDs) are two different types of data storage devices used in computers. They have very different underlying technologies and characteristics. This article provides a detailed comparison between HDDs and SSDs in terms of their technology, performance, durability, power efficiency, capacity, and cost.
HDD Technology
A hard disk drive uses rapidly rotating magnetic platters to store data. Platters are thin circular disks made of non-magnetic material, usually aluminum alloy or glass. At least one platter is installed in a HDD, but there are often multiple platters stacked on top of each other in the enclosure. Platters rotate at very high speeds, typically 5400 rpm to 15000 rpm. HDDs use electromagnetic read/write heads that float just above the platters on an air cushion. The heads can access data tracks on both sides of each platter. Data is written by magnetizing tiny areas of the platter surface. It is read back by detecting the magnetization. HDDs use moving parts including the platters, spindle motor that rotates the platters, actuator arm that holds the heads, and voice coil motor that moves the arms. All these mechanical components make HDDs more prone to failures and damage from shocks.
Main Components of a HDD
- Platters – Stores data magnetically
- Read/write heads – Reads and writes data on the platters
- Spindle motor – Spins the platters
- Actuator arm – Holds and moves the heads
- Voice coil motor – Controls the arm motion
- Control circuitry – Coordinates all operations
- Casing – Sealed enclosure to protect internal parts
SSD Technology
A solid state drive uses integrated circuits to store data persistently. It contains one or more flash memory chips that retain data even when power is turned off. Flash memory cells store data in an array of transistors called floating-gate transistors. Applying a charge to the floating gate traps electrons, changing the threshold voltage required to allow current to flow through the transistor. The cell has two states – programmed and erased, which correspond to binary 1 and 0. SSDs have no moving mechanical components. They use a controller chip to manage all data reading and writing operations. The controller interfaces with the host computer and manages the flash memory addressing, data transfer, wear leveling, bad block mapping, error correction, encryption, etc.
Main Components of a SSD
- NAND flash memory – Stores data in cells
- Controller – Manages memory operations
- DRAM – Buffers data for faster access
- Host interface – Connects to computer
- Firmware – Has algorithms for memory management
Performance Comparison
SSDs significantly outperform HDDs in almost every parameter because they have no moving parts and use NAND flash memory to store data. Some key performance differences:
Access Times
SSDs have much lower access times due to absence of physical moving parts. HDDs require time for the platter to spin and the head to move, adding a delay for each read/write. Typical numbers:
| Metric | SSD | HDD |
| Sequential Read Speed | 500 – 550 MB/s | 100 – 200 MB/s |
| Sequential Write Speed | 500 – 550 MB/s | 100 – 200 MB/s |
| Random Read Speed | Up to 600,000 IOPS | 100 – 200 IOPS |
| Random Write Speed | Up to 600,000 IOPS | 100 – 200 IOPS |
| Seek Time | 0.1 ms | 2 – 5 ms |
Lantencies
The near zero seek time and absence of rotational latencies gives SSDs a huge advantage. Some typical numbers:
| Metric | SSD | HDD |
| Average Seek Time | 0.1 ms | 9 ms |
| Average Latency | 0.05 ms | 4.17 ms |
| Maximum 4K QD1 Latency | 0.26 ms | 7.14 ms |
Durability
SSDs are far more shock and vibration resistant than HDDs because they lack fragile moving parts. Dropping an SSD has no effect on data integrity while dropping a HDD can damage the heads and platters. HDDs also underperform in extreme cold or hot environments. SSDs can withstand a much wider temperature range. However, flash memory cells in SSDs wear out after a large number of write cycles. HDDs do not face this limitation. The durability of an SSD is determined by the number of write cycles its cells can sustain before they can no longer reliably store data. A parameter called TBW (terabytes written) provides this endurance metric for SSDs.
SSD vs HDD Lifespans
| Metric | SSD | HDD |
| Shock Resistance | Very high – No moving parts | Moderate – Sensitive heads and platters |
| Vibration Tolerance | High – Not affected | Low – Head crashes possible |
| Temperature Range | -40°C to 85°C | 5°C to 55°C |
| Altitude Tolerance | Up to 30,000 ft | Up to 10,000 ft |
| Expected Lifespan | About 5 years | About 5 years |
| Longevity Limiting Factor | Flash memory wear out | Mechanical failure |
Power Efficiency
SSDs consume much lower power than HDDs because of absence of energy-hungry mechanical parts. The spindle motor and actuator arm motors in HDDs take up significant power. SSDs only need to power the flash memory chips and controller IC. This gives SSDs a clear advantage for mobile devices where battery life matters. Some comparison numbers:
| Metric | SSD | HDD |
| Active Power (Read/Write) | 1.5 – 2.5 W | 6 – 11 W |
| Idle Power | 0.2 – 0.5 W | 3 – 6 W |
| Power Factor | 0.95 – 0.98 | 0.55 – 0.65 |
Capacity
HDDs can offer much higher data storage capacities compared to SSDs. While SSD capacities have been steadily increasing over the years, HDDs still retain a significant advantage. Current HDDs go up to 20 TB for desktops and 10 TB for mobiles. 2.5″ SSDs only go up to 8 TB and M.2 SSDs up to 4 TB. The maximum capacity offered by HDDs continues to be much higher than SSDs. However, for most consumer and light business workloads, SSD capacities are more than sufficient.
Highest Capacities Available
| Drive Type | Maximum Capacity |
| 3.5″ HDD (Desktop) | 20 TB |
| 2.5″ HDD (Mobile) | 10 TB |
| 2.5″ SATA SSD | 8 TB |
| M.2 NVMe SSD | 4 TB |
Cost Comparison
SSDs are costlier than HDDs in terms of cost per gigabyte. For the same capacity, SSDs are around 4-5 times more expensive than HDDs. The main reason is the higher cost of NAND flash memory compared to magnetic platters. However, the price gap is narrowing down over time. The cost per GB of SSDs has fallen drastically while HDD costs have stayed stagnant. For mainstream home and office computing, HDDs continue to offer better value for money in terms of storage capacity. But for applications like gaming and content creation that need higher performance, SSDs are worth the premium.
Average Cost per GB
| Drive Type | Cost per GB |
| 3.5″ HDD | $0.03 |
| 2.5″ HDD | $0.04 |
| SATA SSD | $0.08 – $0.12 |
| M.2 NVMe SSD | $0.12 – $0.15 |
Conclusion
In summary, HDDs and SSDs take completely different approaches to storing data, resulting in significant performance and characteristics differences. HDDs are mechanical devices using rotating platters while SSDs have no moving parts and use NAND flash memory. SSDs are faster, more power efficient, and more durable but HDDs have the advantage in storage capacity and price per GB. For most everyday users, HDDs continue to provide the best value. But SSDs are preferred for applications needing higher speed and responsiveness. As NAND flash memories mature and become more affordable, SSDs are expected to gain more market share vs HDDs in the future.