Fusion Drive vs SSD vs HDD

By Kevin | Posted to Wiki on Sep 27th, 2020 | User rating:

Summary: This article has a detailed record of the fusion drive, SSD, and HDD in their definition, working principle, components, and features. The bonus part will tell you something little-known to people of the fusion drive. No doubt you'll be a hard drive doctor to choose the best drive when you have finished reading this article and plan to play with your Mac.

Table of contents
Part 1: Rationale introduction of fusion drive, SSD, and HDD
• SSD (Solid State Drive)
• HDD (Hard Disk Drive)
• Fusion Drive
Part 2: Fusion Drive vs SSD vs HDD, for your Mac, which is the best choice?
• SSD home court: Large-sized as well as frequently used files
• HDD home court: Small-sized and occasionally used files
• Fusion drive home court: Compromise for both speed and capacity
Bonus Part of the fusion drive
• Assembly and disassembly of the fusion drive to a computer
• Maintenance of the fusion drive
• Some anecdotes of the fusion drive

Part 1: Rationale introduction of fusion drive, SSD, and HDD

SSD (Solid State Drive) The SSD basically uses the flash memory to store data in the computer so that it has the fastest speed when reading or writing data, but compared to the HDD, it’s kind of expensive and short-life. Read more about the SSD...
HDD (Hard Disk Drive) The HDD is the traditional hard drive with a platter and a read-write head. It’s works based on the electromagnetic induction. Compared with the SSD, the HDD is cheaper and safer, so it is always as the data base used to store large sized data. Read more about the HDD…
Fusion Drive Fusion Drive combines the performance of flash storage with the capacity of a hard drive. Presented as a single volume on your Mac, fusion drive automatically and dynamically moves frequently used files to Flash storage for quicker access, while infrequently used items move to the high-capacity hard disk. Read more about the fusion drive...

1. SSD (Solid State Drive)

(1) Definition of the SSD

The hard disk that is made of the solid-state electronic memory chip array and uses the NAND flash (a kind of flash memory) as the permanent storage medium to store data is SSD.

(2) Components of the SSD


An SSD basically consists of the control unit (the controller), storage unit (NAND flash memory whose main component is flash particles), and cache unit (cache chip). Different from the HDD, the SSD is completely composed of electronic chips and circuit boards without any mechanical part, and that is everything here I talk about the SSD is its logical structure.

• Controller: The controller to an SSD is something like the CPU to a computer. As the kernel part of an SSD, the controller basically has two functions: adjust the load among all flash chips at a reasonable way and bear the task of being the data transfer station by connecting the external SATA interfaces with the flash chips. The controller chips of different brands are always far away from each other in performance.

• NAND Flash Memory: The SSD makes use of the flash memory which uses the flash particles as its storage unit to store the data and takes the place of the mechanical disc and has become the dominator of the storage medium now. It shows up in SSD because this kind of storage medium can keep the stored data alive even if the electricity is cut off. As the flash particle is the trump card in the SSD, it's necessary to know some basic knowledge of it.

The flash particles now in use can be divided into 4 specifications in terms of the electronic unit density in the NAND flash:

ssd flash particles

SLC (Single-Level Cell): Its voltage barely changes when written in data. It is featured with a considerable lifespan, a numbered read-write times over 100000, and the highest cost, which is often seen in high-end products in big companies.

MLC (Multi-Level Cell): it is capable of storing more than a single bit of information compared to an SLC which can store only one bit per memory cell. It has a lifetime of about 1,000 to 10,000 program/erase cycles, with a higher power consumption and lower cost compared to SLC. Recommended users group: professional people who work on storage or those people who are particular about the drive durability.

TLC (Triple-Level Cell): A type of NAND flash memory introduced by Toshiba in 2009 that stores three bits of information per cell. It was first seen in Samsung's 840 Series SSDs and mainly benefits from the still but higher storage density and the lower cost. Attention, now most of us ordinary people are using the TLC SSD.

QLC (Quad-Level Cell): QLC, introduced by Toshiba and SanDisk in 2009, referred to cells that can have sixteen voltage states. It has the lowest cost and the highest density but the worst performance and durability. Recommended users group: large-sized games collectors and keepers of data warehouse.

PLC (Penta-Level Cell): Though it does not exist yet for consumers right now, this future particle SSD drive which can write 5 bits per cell is on the way. Toshiba mentioned PLC drives in late August 2019, and Intel the following month. PLC drives should be able to pack even more capacity into SSDs.

• Cache: It is the last part that can attract people perhaps because the cache will get empty when the computer bumps into an outage. Not all SSD has this part, for it's not a dominating factor to affect the SSD performance. Therefore, the Pluto is kicked out of the Nine Planets by some big SSD companies! However, it's quite something! Cache means data, especially small-sized and frequently used random data can be quickly read when you have a cache part inside your SSD, and which used to be the maximum focus in the old years of HDD. Into the new era of SSD, although the importance of the cache has been no longer what it was years ago, it is now still active in promoting the reading speed of the fragmented files in your SSD. Moreover, you'll find your SSD will run slower when you have run out of its cache space!

If the exchange between the cache and the SSD runs slower than the cache consumption, the cache space will run out.

(3) Working principle of the SSD

The outline between the computer and the SSD can be simply described as bellow:

ssd computer work outline

However, the concrete relationship is far more complex than that:

In the computer, on the one hand, the Host (any computer instruction that is aimed at data access or transmission) accesses to the SSD via the LBA (Logical Block Address), each LBA stands for a Sector, and the operating system usually accesses to the SSD at every 4KB unit. The basic unit where the Host accesses to the SSD has its term, Host Page.

ssd logical and physical mapping

On the other hand, inside the SSD, the SSD controller accesses to the flash particles on the basis of Flash Page, which is also named the Physical Page. Every time the host writes a host page in, the SSD controller will seek for a Physical Page to write the Host Data (host data means the data that the host instruction writes about), which actually forms a mapping relationship and this mapping relationship is recorded by the SSD. As long as this mapping relationship exists, the SSD knows exactly where to find the wanted host page from the flash particles.

Besides, there is a Map Table to record all the mapping relationships in the SSD. Each time a mapping relationship is born, it will join (the data is written for the first time) or rewrite (the data is written repeatedly) the map table. When a host page is being read, the SSD will at first search for the physical page of it in the map table and then go to the flash particles to seize the target data.

ssd map table

Of course, all those things that could go fine are premised on the specialty of the SSD, which is that the SSD apportions different amount of electronics in the gate of the storage unit transistor to change the electrical conductivity, or rather, the open-close states of the transistor in order to recognize and record the different states.



① Fast speed of read-write

② Shake-resistant and hard to be physically broken

③ Broad range of working temperature, from 40 degrees Celsius to 85 degrees Celsius

④ Low noise

⑤ Heat slow while cool fast

⑥ Light-weight


⑦ High price

⑧ High power consumption

⑨ Low capacity

⑩ Stall speed when used for a period

⑪ Read disturbance (it can lead to hopeless data loss)

⑫ Nearly impossible to save data from broken flash particles

⑬ Low life expectancy

PS: What if your SSD doesn't show up in the Windows computer? Click to find the anwser!

2. HDD (Hard Disk Drive)

(1) Definition of the HDD

The old-school storage device that uses mechanical platters and a moving read/write head to access data is HDD.

(2) Components of the HDD

hdd physical structure

As far as the physical structure, the HDD consists of the Voice Coil Actuator, Actuator arm, Read/write head, Platter, and some other nuisances, but among them, the core parts are just the Platter and the Read/write head whose cooperation is fundamental to make the function of the HDD into effect.

As far as the logical structure, the HDD is basically composed of the Platter, Track, Cylinder, and the Sector.

hdd logical structure

Platter: It is commonly made of aluminum, and both sides of it are able to be used to store the data. But not all manufactures would like to make use of both sides for the sake of simplifying the read-write head structure. Each platter side is with a fellow read-write head.

Track: When the platter spins, if each read/write head keeps its position, then each head is going to line out a circular trace, and that's the track.

Sector: Each track is divided into many arcs, and these arcs are sectors. The most common capacity of a sector is 512 bytes.

Cylinder: As the platters are always paralleled and the same size, thus each platter has the same quantity of track. Each platter gives its tracks a number from outside to inside. The outermost track is numbered "0", and every time you add one track towards the center means the track number plus "1". Those tracks numbered with the same number in each platter form a cylinder.

(3) Working principle of the HDD

1) Data access to the HDD

When the computer needs to read some data from the HDD, the operating system will pass down the HDD logical address to the HDD, and then the HDD control circuit will translate the logical address to the physical address so that the HDD can make sure which sector the wanted data is in. When the wanted data is found, to read it, the HDD will put its read-write head above it, and this process covers 2 steps:

① Find the cylinder of the wanted data to latch the track.

② Spin the wanted sector until it is right under the read-write head.

2) Data read and write of the HDD

How is the data read and written by the computer when the wanted data has been found? Well, the HDD reads or writes data from head to toe and outside to inside, and the closer the track is to the center, the slower the track is read and written. Depending on the request (read or write data) from the computer, the HDD controller will make up its mind to convert to write the circuit (write data) or just read the data and trailer record (a record that follows a group of related records and contains data relevant to those records). If it is data read, before going for the next sector, the HDD controller has to calculate the current data's Error Correcting Code (ECC) and compare it with the existed ECC; if it is data write, then the HDD controller will calculate the ECC and store the ECC with the current data.

Features of the HDD


① Large storage space

② Mature technology

③ Lower cost

④ Better data rewrite ability

⑤ Long lifespan

⑥ Lost data retrievable


⑦ Read/write slow

⑧ Consume more power

⑨ Heat up fast

⑩ Noisy

⑪ Poor shake/beat resistance

3. Fusion drive

(1) Definition of the fusion drive

A hard disk that combines the performance of the SSD and the capacity of the HDD is a fusion drive. Now, the fusion drive, a technology that is developed by Apple, can be seen on a Mac computer.

(2) Components of the fusion drive

If you are using the Mac fusion drive, then the fusion drive done by the manufacture consists of an HDD and a Flash chip.

imac fusion drive

And when you go to the Disk Utility, you'll see what it is like as below:

fusion drive disk utility

This means the manufacture in the factory inlays flash particles into a traditional HDD that is equipped with the platter, read-write head, and etc.

However, if you want to make a fusion drive on your Mac by yourself, then you need a Mac HDD and a Mac SSD. Attention, it is a Mac HDD and a Mac SSD in a Mac computer! That's because nearly all Apple-branded products can only marry with their Apple compatriots for the vast monopoly profits!

self-made fusion drive

The Disk Utility will give you such information when you finished the work:

self-made fusion drive disk utility

As a software, the Disk Utility can only tell you the result of the SSD and HDD combination. Therefore, the second kind of the fusion drive is almost displayed the same as the first kind of fusion drive. But physically, it is indeed made by two separate parts: an SSD and an HDD. When there's something going wrong with it, like file system failure, or when you forcibly split them up through some technical measures, the Disk Utility will show you such a picture:

fusion drive splitup

You can see both parts clearly!

(3) Working principle of the fusion drive

Generally speaking, the fusion drive combines the performance of flash storage with the capacity of an HDD. Presented as a single volume on your Mac/Mac, fusion drive automatically and dynamically moves frequently used files to flash storage for quicker access, while infrequently used items move to the high-capacity HDD. In fact, the fusion drive has no unique and fancy working principle as its working principle is just SSD working principle + HDD working principle. I guess you may wonder how can a single volume average the SSD part and the HDD part? The answer is its algorithm, the most important element inbuilt with a fusion drive to let us know how leading it is, and it is the algorithm depending on your used file storage habits and its deep learning AI ability that assigns and coordinates the storage tasks between the HDD part and the SSD part.

(4) Features of the fusion drive


① Faster read-write speed than HDD

② Safer than HDD

③ Broad capacity

④ Great response speed

⑤ Intelligent learning


⑦ Low small files read-write speed

⑧ Not so much cheaper than the SSD

⑨ Low market share and elbowed out by the SSD and SSHD (Solid State Hybrid Drive)

⑪ Extremely difficult to recover the lost data

⑫ Quite limited SSD capacity

⑬ Immature technology

Part 2: Fusion Drive vs SSD vs HDD, for your Mac, which is the best choice?

• SSD home court: frequently used files, but not particular about the capacity

The common SSD read/write speed in 2020 can be easily at 500MB/s, which means a medium-sized software like, PS, AU, AI, AE can be read and loaded no more 10 seconds. Think about this scene that years before you wait for the PS, but years later the PS waits for you, and that's how amazing the SSD is! Also, when you press the startup key of a computer with an HDD and plan to pee, you'll find yourself taking a dump whereas when you press the startup key of a computer with an SSD and plan to pee, you'll find yourself playing the Fortnite right away! Its speed as well reflects in compressing/decompressing files. You can even count how many seconds it cost to do that. As for those people who are working with the complex design software that needs complex operations like complex Adobe family, Lightroom, Logic Pro X, and TinkerCAD, I suppose none of them would like to spend time and money in watching the crushed workspace and a wait-for-response window. Attention please, when the capacity isn't that a demanding factor, the more frequently you use the files, the more necessarily you should put them in the SSD!

• HDD home court: large-sized but not so frequently used files

The HDD is not out of data only if you there is someone who needs storage capacity over 4T to store the infrequently used data (most of the common SSD for the ordinary people can store files no larger than 4T for the time being) and is not always after the I/O performance like music listening, picture browsing, and video watching. The HDD shows its privilege off incisively and vividly in the field of low unit storage cost for that of the same capacity, the SSD storage unit cost is four times as much as the HDD storage unit. Moreover, once bought SSD could lose its youth and speed or even some fatal data when used for years whereas no similar worries needed about an HDD, because the HDD is relatively long-life, stable, safe, and always serves loyally for his lord with the most silent sigh of the spinning platter and it will always say:

"Let me keep your most important things."

• Fusion drive home court: Compromise for both speed and capacity

"That' it baby! That's it!"

To satisfy these people who want to enjoy both the speed of the SSD and the capacity of the HDD on Mac, the fusion drive technology is first implemented by Apple on October 23, 2012, with the first supporting products being two desktops: the iMac and Mac mini. So, what's the best usage of the fusion drive?

As the SSD part of a fusion drive is quite limited, then all you need to do is put the small-sized and frequently-used data in the SSD part to enjoy its speed, like the operating system files (it's a must), office software files, and the browser profiles while place the large-sized and infrequently used data like downloaded movies, OS files backup, old pictures, and songs in the HDD to enjoy its capacity.

Bonus Part of the fusion drive

• Removal and installation of the fusion drive

Except for Mac and other few products, Apple has nearly sealed all its computers' hard drive to the motherboard, like MacBook Pro and MacBook Air, and made its unique disk interface which is unworkable with non-apple hard drives since 2012. So, the Mac survived to free remove and install its hard drive. But unfortunately, the 2020 27inch Mac has been done the same things, which marks Apple has finished its hard drive sealing mission on all Apple computers. So, think twice about the fusion drive scheme when you plan to buy a Mac, asking yourself how you would like to use it because the hard drive is non-upgradable and irreversible.

• Maintenance of the fusion drive

The main parts, including the motherboard, CPU, RAM, hard drive, GPU, and power source, of the Mac branded products have a two-year warranty plus a 90 days tech support. If your Mac fusion drive needs repairing when it's beyond the warranty period, well, man you get in trouble. A national average price range for hard drive repair or replacement is about $159-$459.

• Some anecdotes of the fusion drive

① One thing you ought to know is that the Mac fusion drive is a kind of hybrid hard drive and nearly has all of the features that an ordinary hybrid hard drive has, but it doesn't mean the Mac fusion drive completely equals to a hybrid hard drive because the Mac fusion drive is much more superior than any other hybrid hard drives in the algorithm. The more power algorithm gives birth to the fusion drive the faster read and write speed.

② Only a hybrid hard drive that is made by Apple or made and used in a Mac can be called a fusion drive. The fusion drive is unique to Apple.

③ The fusion drive is in-built when you order a Mac, and usually there are four selections: 1T fusion drive, 256G SSD, 512G SSD, and 1T SSD.

④ The fusion drive doesn't have a bright future because it is not as good as what it seems to be. Because when the SSD part runs out, you can't any more enjoy the speed for newly downloaded files or when you mix the files that should be placed in the SSD with the files that should be put in the HDD, it will end up being no blessing for you to enjoy both of the virtues. Under this situation, the fusion drive is not worth the compromise for both the higher cost than the HDD and much lower speed than the SSD. By the way, the price of the SSD has fallen dramatically in recent years, and the trend is on and on as well as its quality has been far more prompted compared with that of a couple of years ago. Personally speaking, it's not a wise choice for most of the ordinary people in 2020 because for example, the iMac failure of either the SSD part or the HDD part can sentence a death penalty that all your data is gone forever.

Now that SSD has become the Tik Tok in the field of the hard drive in 2020, how can other types of hard drive survive? What will the world of hard drive be like in the near future? If you have any interesting ideas or good advice, you're welcome to leave your comments right below or communicate with us through the mailbox. Our email: [email protected] Thank you for your reading!

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This article really helped me a lot, thanks!

2020-09-29 03:43:19