Flash storage and hard disk drives (HDDs) are two very different technologies for digital data storage. Flash storage relies on flash memory chips while hard drives use magnetic disks to store data. At first glance, it may seem impossible for a hard drive to utilize flash storage. However, some hybrid drives have emerged that aim to get the best of both worlds.
The Basics of Flash Storage
Flash storage devices, also known as solid state drives (SSDs), use flash memory chips to store data. Flash memory is a type of electronically erasable programmable read-only memory (EEPROM). It got its name because data is erased from the chips in “flashes.”
Each flash memory cell resembles a standard MOSFET transistor with a floating gate inserted between the control gate and substrate. The floating gate traps electrons, allowing data to be stored. Flash memory cells are typically arranged in blocks. Data can be read or written at the byte level, while erase operations happen in entire blocks.
Compared to mechanical hard drives, flash storage offers much faster access speeds, better durability and shock resistance. However, flash storage used to be much more expensive per gigabyte than HDDs. Over time, the cost has decreased considerably, making flash a viable option for more and more applications.
Advantages of Flash Storage
- No moving parts, allowing for better shock/vibration resistance
- Much lower access latency and faster read/write speeds
- Higher reliability and longer lifespan
- Lower power consumption
- Compact and lightweight
Disadvantages of Flash Storage
- More expensive per gigabyte than hard drives
- Limited number of erase cycles per memory cell
- Slower erase speeds compared to read/write speeds
- Susceptible to read disturbs and write fatigue
How Do Hard Disk Drives Work?
Hard disk drives rely on magnetic recording to store and retrieve digital data. A typical HDD consists of one or more rotating magnetic platters inside an air-sealed enclosure. Each platter surface is coated with a magnetic material and has a read-write head positioned nanometers above it.
As the platter rotates at high speed, the head can magnetize tiny regions on the platter surface to represent binary data. The regions maintain their magnetic orientation even when the disk stops spinning, allowing data to be stored persistently.
When reading data, the head detects changes in the magnetic fields emanating from the platter surface as it passes by. These magnetic field changes are decoded into 1s and 0s. HDDs use electromechanical components like actuators to properly position the heads for precise reading and writing.
Advantages of Hard Drives
- Low cost per gigabyte of storage
- High maximum capacities available
- Mature technology with decades of refinement
Disadvantages of Hard Drives
- Slower access speeds due to mechanical/magnetic latency
- More fragile with lower shock tolerance
- Higher power consumption, noise and heat output
- Heavier and bulkier compared to flash options
Hybrid Hard Drives
Hybrid hard drives (HHDs) or solid-state hybrid drives (SSHDs) combine flash storage with traditional HDD technology. This allows manufacturers to deliver drives that offer some of the speed and durability benefits of SSDs while keeping costs closer to those of HDDs.
A hybrid drive typically includes a high capacity hard disk platter along with a smaller SSD component, ranging from 8GB to 32GB. Frequently accessed data like boot files and commonly used applications are cached on the flash storage for quicker loading. An on-board controller manages data transfers between the SSD and HDD portions seamlessly.
When the computer needs a file, it first checks the faster flash storage before resorting to the slower mechanical disk. The controller may also proactively move data from the HDD to the SSD based on usage patterns. This way, booting and launching apps happens much quicker even though bulk storage still relies on the HDD.
By adding just a small amount of flash, hybrid drives can get a significant improvement in performance. Of course, they cannot reach the speeds of pure SSDs, especially when dealing with large sequential transfers rather than small random I/O. Still, hybrid HDDs offer a compelling balance for many. Almost every HDD manufacturer now offers hybrid drives as part of their product lineup.
Examples of Hybrid Hard Drives
- Seagate FireCuda
- WD Black SSHD
- Toshiba SSHD
Can a Regular Hard Drive Be Upgraded to Use Flash?
In most cases, it is not possible to upgrade an existing, traditional hard drive to use flash storage. However, there are a few approaches that may seem like adding flash to a hard drive in certain situations:
Hybrid Hard Drive Upgrade Kits
Some companies offer hybrid hard drive upgrade kits. These include a small SSD along with enclosure adapters and software/cables. You remove your original hard drive, install it into the adapter case along with the SSD. The end result connects to your computer like a normal hard drive but now has some flash storage to speed things up.
While this approach lets you gain some benefits of flash, it is not the same as truly integrating flash memory within the hard drive itself. The SSD remains as a separate, external component. The speed gains may also be smaller compared to a purpose-built hybrid drive.
Flash Caching Drives
Another option is a flash caching hard drive. These drives add an SSD directly onto the circuit board of the HDD. The flash chips don’t replace or augment the magnetic disk itself. Rather, they serve solely as a cache to improve throughput.
Reads and writes still go to the HDD platters, but a subset of that data is copied to the flash cache as well. Subsequent requests can be satisfied from the cache for much faster access. As the cache has limited space, it only stores frequently and recently used data.
Flash caching drives can boost speeds for certain workloads. However, they lack the sophistication of hybrid hard drives that can proactively cache data based on predictive algorithms. And adding on-board flash increases the cost compared to standard HDDs.
M.2 or U.2 Flash Storage Slots
Some high-end desktop workstations and servers support modular drive bays that can house either HDDs or SSDs. For example, U.2 bays allow hot swapping of either 2.5″ HDDs or SSDs. M.2 slots can also accommodate both SATA SSDs and slower M.2 HDDs.
With these systems, you could theoretically start with an HDD installed and later swap in an SSD. Of course, this is not converting the internal hard drive itself to use flash. But to the end user, it may seem like “adding” flash storage to a machine that originally only had HDDs.
Reasons Flash Storage May Not Be Desirable
While flash storage has many benefits, HDDs remain very popular due to some continued advantages. This means consumers and businesses may prefer to stick with traditional hard drives in many use cases.
Cost Per Gigabyte
The biggest reason HDDs maintain dominance in PCs and data centers is their low price per gigabyte. HDD capacity continues to grow rapidly, keeping costs down. High-capacity HDDs up to 16 TB can be purchased for a reasonable price. Equally spacious SSDs remain outside most consumers’ budgets.
Maximum Capacities
Currently, the highest capacity flash drives max out at around 100 TB for enterprise models. HDDs can offer much more, with commercial 20+ TB drives available. Applications requiring massive, affordable storage still rely on mechanical disks.
Sequential Performance
Although flash storage dominates on random I/O, HDDs can deliver better sequential performance when reading/writing large, contiguous blocks of data. Use cases like high-resolution video editing still leverage this strength of hard drives.
Less Wear From Rewrites
Flash storage has a finite number of program-erase cycles before cells wear out. HDDs based on magnetic recording don’t suffer wear in the same way. Applications requiring very high write endurance over time still favor hard drives.
Maturity and Incumbency
Hard disk technology has been refined over decades. Manufacturing and supply chains are optimized for large volume production. Many legacy systems rely on HDDs by design. For various reasons, switching to flash may not be viable in the near term despite some benefits.
The Future of Storage
Looking ahead, flash storage will likely continue displacing hard drives in more and more areas as costs drop. However, HDDs will also evolve and retain an advantage for very high capacities. Other new technologies may emerge to complement flash and HDDs in specific applications.
Some future directions include:
- Hard drives with microwave-assisted magnetic recording (MAMR) and bit-patterned magnetic recording (BPMR) to increase capacities
- 3D NAND flash to continue density improvements
- Faster interfaces like PCIe 4.0, USB 3.2 Gen 2×2, and SATA Express
- Quad-level cell (QLC) and penta-level cell (PLC) flash to lower cost
- Persistent memory like Intel’s 3D XPoint to offer DRAM-like speeds
- New technologies like racetrack memory, holographic storage, and DNA digital data storage
In the end, both hard drives and flash storage have unique strengths that will ensure they remain useful into the future. Hard drives cannot directly use flash, but hybrid drives offer a bridge between the two technologies.
Conclusion
While hard disk drives and flash storage are fundamentally different, hybrid drives allow some flash memory to augment traditional HDDs. This provides much of the speed of solid state drives without completely abandoning the cost advantage of hard drives. However, limitations exist based on the small amount of flash in hybrid drives.
It is not possible to equip a standard hard drive directly with flash. But some upgrade kits and flash caching drives can approximate the experience of “adding” flash to an HDD. In servers, support for swappable drive bays enables starting with HDDs and later transitioning to SSDs.
Hard drives will continue playing a key role thanks to advantages in price, max capacity and maturity. But flash adoption will expand in markets where its strengths outweigh the benefits of magnetic HDDs. Both technologies will advance and be suited for different applications.