Hard disk drives (HDDs) have been the primary form of computer data storage for decades, but they have some drawbacks compared to newer solid state drives (SSDs). The two main disadvantages of HDDs are:
1. Slower performance
HDDs rely on mechanical moving parts like read/write heads and spinning platters. This makes them much slower than SSDs which use microchips and have no moving parts. The maximum sequential read/write speed for consumer HDDs is up to 210MB/s, while SSDs can reach over 500MB/s.
This speed difference is very noticeable when booting up your computer, launching programs, saving files, or loading game levels. HDDs are a bottleneck for any computer trying to achieve fast performance.
2. More fragile/ prone to failure
The mechanical nature of HDDs also makes them more fragile than solid state drives. Spinning disk platters and moving read/write heads mean there are many precision parts that can fail. And due to their internal movements, HDDs are susceptible to damage from drops, vibrations, magnetism, extreme temperatures, and physical shock.
SSDs have no moving parts so they are more sturdy in harsh environments. They can withstand accidental drops that would likely damage HDDs.
Over time HDDs experience mechanical wear and tear that causes their performance to degrade gradually. The average HDD lifespan is 3-5 years under regular use. SSDs can last over 10 years thanks to their simpler electronic storage method.
Hard Disk Drive Components and Operation
To fully understand the disadvantages of HDDs, it helps to first look at what’s inside them and how they work:
Disk platters
The core part of a hard drive is one or more flat, circular platters made of non-magnetic material, usually aluminum or glass. These platters are coated with a thin magnetic material that is used to store data. Most consumer HDDs have 1-4 platters stacked on top of each other in the drive enclosure.
Read/Write Heads
Read/write heads are the devices that actually read and write the magnetically encoded data on the platters. There is one read/write head for each platter surface (top and bottom). The heads float just above the platters on an air bearing generated by the disk’s rotation.
Spindle
The spindle is the rod that passes through the center of the stacked platters and connects them to the drive motor. This allows all the platters to spin together at a constant rate, typically 5400 RPM or 7200 RPM.
Actuator arm
The actuator arm holds the read/write heads and allows them to move back and forth across the platters as needed to access different data tracks. It has a pivot point and voice coil that controls its positioning.
Interface and electronics
The interface manages communications between the drive and computer. SATA and USB are common interfaces for connecting HDDs. The electronics board also controls the spindle motor and actuator arm. It coordinates all read/write operations and data transfers.
How Hard Drives Store Data
Data is stored on the magnetic platters in concentric, circular tracks. Within each track, data is stored in sectors – small pie-shaped wedges representing individual bit storage areas.
As the platters spin, the read/write heads can change position to access different tracks. The actuator arm moves the heads toward the edge or center of the platters. By pivoting the heads to alter their radial position, they can reach any track location.
Each track is further broken down into sectors. Common sector sizes are 512 bytes or 4KB. 512 bytes was the established standard for many years, while 4KB is a newer standard allowing for greater storage densities. The drive electronics utilize the sector, track, and platter locations to accurately read and write data.
Magnetic storage
So how is the binary data actually stored magnetically? Each platter surface is magnetized in small sub-micrometer regions representing 0s and 1s. These magnetized grains are arranged in a circular pattern in each track. One bit is stored in each magnetic region. The read head senses the magnetization direction of each tiny spot to read the 0s and 1s.
Data is written by the heads flipping the magnetization of spots to the desired direction. The magnetic regions retain their magnetization state even when the disk platters stop spinning, allowing long term persistent storage. This is why HDDs can store data even when powered off.
Areal density
The main goal of HDD advancement has been cramming more data bits into the same disk space. This is measured by areal density – the number of bits that can be stored per square inch of platter surface. Today’s HDDs achieve around 1 terabit (1 trillion bits) per square inch. This allows large multi-terabyte capacities to fit in small drive enclosures.
HDD areal density has historically increased at a rapid rate of around 30-40% per year, enabled by technologies like perpendicular recording and shingled magnetic recording. But recent years have seen slowing density gains as current technology bumps up against the physical limits of magnetic storage.
Disadvantage 1: Slower Performance
Now that we’ve looked inside hard drives, we can better understand their major disadvantages compared to solid state drives. First and foremost is their slower performance. Let’s examine the reasons HDDs are slower:
Mechanical latency
The very nature of spinning disk platters introduces latency since the disks must spin until the required location rotates under the head. On average, the desired sector will be halfway around the disk from the head’s current position, adding a rotational delay of several milliseconds.
This mechanical rotational latency does not exist for SSDs since they have no moving parts. Data can be accessed nearly instantly.
Seek time
Seek time refers to the delay for the actuator arm to radially position the read/write head over the correct track on the platter where data needs to be read or written. Seek time averages around 10 ms for modern HDDs.
Again, SSDs do not endure any physical seek time since electronic solid state memory has no concept of physical “tracks”.
Transfer rate
The maximum sequential transfer rate of HDDs is limited by the speed of mechanical components like the platter rotation speed and actuator arm velocity. Current consumer HDDs typically max out at around 210 MB/s transfer rate for sequential large block data transfers. This is over 3x slower than some SATA SSDs which exceed 500 MB/s transfer rates.
File opening and loading
The combination of latency, seek times, and transfer rate limitations significantly slow down HDD operation. This is very noticeable for computer users when opening programs or files. Software and game levels can take much longer to load from HDDs compared to SSDs.
Boot time
Booting up a computer with the operating system installed on a hard drive can take minutes, versus just seconds for solid state drives. This is again caused by the mechanical delays inherent to HDDs.
Fragmentation
File fragmentation happens when pieces of files get scattered in different locations on the hard drive platters. This occurs naturally over time as files are modified, deleted, and overwritten. Fragmentation leads to significantly slower HDD read/write speeds because the actuator arm must frequently seek between the disjointed file fragments.
SSDs don’t have fragmentation issues due to their simplistic block storage method that has no concept of file locations or fragmentation.
Disadvantage 2: More Fragile and Prone to Failure
In addition to slower speed, HDDs are also more mechanically fragile than solid state drives due to their moving parts:
Impact and shock
Being dropped or experiencing a physical impact can easily damage HDDs. The precision spindle motors and actuator arms include bearings that do not tolerate shock loads well. Platter scratching is also a possibility if the heads contact the platters. SSDs have no risk of mechanical damage or shaken components from impacts.
Vibration
Constant vibration from sources like heavy equipment, motors, noise, and transportation can negatively affect HDD operation and shorten their lifespan. The mechanical components and gyroscopic forces make HDDs far more prone to damage from vibration than vibration-tolerant SSDs.
Magnetism
HDDs must avoid sources of magnetic fields which can erase data or corrupt the low level formatting on the platters. Anything as benign as an ordinary magnet must be kept far away from HDDs. In contrast, SSDs are utterly immune to any incidental magnetism issues.
Temperature and humidity
Excessive heat or humidity can create issues for some internal HDD components leading to potential data loss or drive failure. SSDs tolerate a much wider temperature range and humidity levels with no adverse effects.
Contaminants
Dust and debris inside the HDD case can lead to a head crash where the read/write head makes contact with the platter surface, destroying data. Clean room assembly practices help avoid contaminants during manufacturing, but over time some external pollution is possible in the drive casing. The fully electronic SSD architecture has no contaminant risks.
Wear and tear
Years of use inevitably causes mechanical wear on HDD parts like the bearings in the spindle motor and pivot arm. This causes gradual performance degradation and eventual failure as components exceed their lifespan. SSDs are not subject to the same physical wearing effects over time.
Failure rate
Research shows that hard disk drives have annual failure rates of around 4%, with a life expectancy around 3-5 years. In contrast, SSDs typically have sub-1% failure rates and can last over 10 years. So the average HDD is far more likely to experience a catastrophic drive failure than a comparable SSD.
Advantages of Solid State Drives
We’ve covered the pitfalls of hard disk drives in detail. For comparison, let’s summarize the major advantages of solid state drives:
Faster access and data transfer
SSDs provide incredibly fast random access speeds of less than 0.1 ms. Sequential data transfer speeds can reach over 500 MB/s compared to ~100-200 MB/s for HDDs.
Rugged and reliable
With no moving parts, SSDs can withstand vibration, shock, magnetism, temperature changes, humidity, and contaminants much better than mechanical hard drives. Lifespans over 10 years are common.
Low power and noise
SSDs consume less idle and active power than HDDs. They also produce no noise since they don’t have spinning platters or moving actuator arms.
Compact and lightweight
2.5″ SSDs weight around 1.5 ounces. 3.5″ HDDs weigh over 1 pound. This makes SSDs better suited for small, lightweight devices.
In summary, choosing an SSD over HDD provides huge performance, reliability, and usability improvements in everything from desktop PCs to enterprise data centers. The only downside of SSDs is higher cost per gigabyte of storage, but the price gap continues to narrow.
When HDDs are Appropriate Despite Drawbacks
With all the benefits of SSDs, you may wonder why hard drives continue to exist. There are a few scenarios where HDDs still make sense:
Very large storage needs
If you need more than 10 terabytes of storage, HDDs are likely the only cost-effective solution. High capacity SSDs exist but are prohibitively expensive for most. As NAND flash costs decline, this advantage of HDDs continues to diminish.
Non-essential storage
If you just need raw bulk storage for backups or archives that don’t need quick access, HDDs are fine. The slower speed is not detrimental for cold storage use cases.
Budget constraints
In cost-sensitive environments like basic desktop PCs or NAS devices, HDDs can provide decent storage capacities at low cost. But even cheap SATA SSDs continue to fall in price and become viable options for more use cases.
Legacy systems
Older computers may lack the interface support or bios compatibility for SSD usage. HDDs are readily available for older SATA and IDE interfaces, ensuring backwards compatibility.
So in select use cases, the mature HDD technology remains a reasonable option. But new PC builds today should almost always opt for SSD boot drives, then add a HDD only if large secondary capacity is needed. The performance benefits of SSDs are just too substantial to ignore for primary storage.
Hybrid Solid State/Hard Disk Drives
Hybrid drives, also known as SSHDs, combine an HDD with a small SSD acting as a cache. A sophisticated algorithm copies frequently accessed data blocks onto the SSD portion for faster access. This hybrid approach aims to provide some of the speed benefits of SSD caching without the large cost.
However, true SSD prices have been falling so quickly that hybrid SSD/HDDdrives have lost popularity. A dedicated SSD boot drive with optional separate HDD is usually the better choice. The hybrid approach made more sense when SSDs were far more expensive than today.
External Hard Drives
Both HDDs and SSDs are available in external enclosures connected over USB, Thunderbolt, or other interfaces. External HDDs serve as a portable method of adding substantial bulk storage capacity to laptops or desktops. Their large capacities and low costs make them useful for data backups, media libraries, and other large datasets.
That said, external SSDs are also growing in popularity thanks to their portability, durability, and transfer speeds faster than any external HDD. Sizes up to 16TB are now available in compact enclosures using USB 3.2 or Thunderbolt 3 interfaces.
So external HDDs still work well for simply expanding storage, while external SSDs provide extremely fast portable solid state storage for external boot drives, video editing, or high speed backups.
| Attribute | Hard Disk Drive (HDD) | Solid State Drive (SSD) |
|---|---|---|
| Data storage medium | Magnetic platters | NAND flash memory chips |
| Sequential access speed | Up to 210 MB/s | Over 500 MB/s |
| Random access speed | 2-10 ms | Below 0.1 ms |
| Reliability | 3-5 year lifespan, 4% annual failure rate | Over 10 year lifespan, <1% annual failure rate |
| Resistance to shock/vibration | Medium – contains sensitive mechanical parts | High – no moving parts |
| Power consumption | 5-10 watts active power | 2-4 watts active power |
| Noise output | Audible noise from spinning platters | Silent operation |
| Maximum capacity | 16TB for consumer drives | 16TB for 2.5″ SSDs |
| Cost per gigabyte | Around $0.03 per GB | Around $0.20 per GB |
Conclusion
While HDDs still retain some niche advantages like very high capacities, SSDs are the clear technology of choice for the vast majority of applications. The speed, durability, power efficiency, and compact size of solid state drives make them superior to traditional hard disk drives in nearly every metric.
With 1TB+ SSDs available at increasingly reasonable costs, HDDs no longer have a compelling price advantage. The future is solid state for computer storage. HDDs will continue fading in popularity as SSD prices fall further.
