I was just reading a question on backups with flash drives, but I was not quite sure why SSDs would be better than flash drives for backups.

To me, solid-state drives look like larger versions of flash drives. But surely there must be something that makes the former much more reliable than the latter.

What's the difference between an SSD and just a regular flash drive? For example, do they both use NAND?

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    Not a definitive answer: speed and wear-levelling technology. On the latter, flash-drive cells have a limited number of times they can be written to; so SSDs are designed with heavier write-loads than are typical flash drives. They achieve this by having redundant writable cells and algorithms to rotate between these cells. Having said that, there's no technical reason flash-drives can't have these. – Otheus May 24 '15 at 23:58
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    And I would like to know (in addition to question) whether there is a difference between NAND flash storage in Android phones and the SSD? – Firelord May 25 '15 at 0:37
  • Differences are and not limited to the quality of the flash memory cells and the memory controller. Usb flash drives typically use the lowest quality flash memory. – Moab May 25 '15 at 13:56
up vote 54 down vote accepted
+50

Both Flash and SSD are based on NAND-based flash memory, which retains data without power, and so can be labelled as Flash memory.

Technologically, the main differences between the two are in :

  1. The underlying technology used to construct the NAND,
  2. The quality of the Flash memory controller,
  3. The computer connector : USB or SATA.

NAND technologies

NAND technologies diverge on two points : Speed and price.

On the one hand one finds MLC (Multi-level cell) which is a memory element capable of storing more than a single bit of information. Most MLC NAND flash memory has four possible states per cell (or even more with TLC), so it can store several bits of information per cell. This reduces the number of required transistors, so reducing size and manufacturing costs, while also reducing speed and increasing the possibility of errors.

On the other hand one finds SLC (single-level cell), where each cell can exist in one of two states, storing one bit of information per cell. This increases the access speed, while also increasing manufacturing costs and electricity usage.

An MLC cell is typically rated at 10,000 erase/write cycles, while an SLC cell might last 10 times that before failing.

Because of these differences, MLC is typically used in slower and cheaper media, accessed typically via USB. A good SSD will use SLC and be costlier, but faster, have a longer life-time and be typically accessed via SATA 2 or 3.

Memory controller

A USB mass storage controller has only a small micro-controller with a small amount of on-chip ROM and RAM.

An SSD controller is much more complicated. The controller is an embedded processor that executes firmware-level code and is one of the most important factors of SSD performance. Some of the functions performed by the controller include:

  • Error-correcting code (ECC)
  • Wear leveling
  • Bad block mapping
  • Read scrubbing and read disturb management
  • Read and write caching
  • Garbage collection
  • Encryption

In a hybrid SSD, the controller will also manage a small classical hard disk.

Connector

A flash stick normally uses a standard-A USB plug that provides the physical interface to the host computer. These can now go up to USB-3 speeds for the more costly models, or USB-2 for the common ones.

SSD technology uses electronic interfaces compatible with traditional block input/output (I/O) internal hard disk drives. Additionally, new I/O interfaces, like SATA Express, have been designed to address specific requirements of the SSD technology. Most SSD cards are typically much faster than classical hard drives.

Summary

A Flash stick typically has less memory capacity, is slower, cheaper and is also less dependable than an SSD.

There are of course always devices that bridge these differences by using compensating technologies.

References :

  • +1 But not all, but most, flash and SSD storage are NAND based – Keltari Jun 28 '15 at 19:23
  • @Keltari Can you give me an example of an actual SSD product that is not using NAND? I am curious, because I have never seen one. What kind of capacity would a NOR based SSD have? I mean I know that NOR based flash memory exists. It's been invented by Intel in 1988. But currently, all SSD drives I know of are using NAND flash, which was invented by Toshiba in 1989. We know that NAND based SSD drives are leading the way, in terms of low cost and high capacity. I don't think NOR based SSD will catch on anytime soon. Thus my question, to have a look at a finished NOR based SSD product. – Samir Jun 28 '15 at 19:58
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    Errata: Intel was first to introduce a NOR based flash chip in 1988, and Toshiba followed in 1989 with its NAND based chip. It was invented earlier than that though, by Dr. Fujio Masuoka and his team at Toshiba, and first presented in 1984. – Samir Jun 28 '15 at 20:15
  • Very nice answer! Took the highest-repped person in SU to write it, too ;) – oldmud0 Jun 29 '15 at 1:37
  • According to the wikipedia article for CompactFlash, they were originally NOR flash, but have since switched to NAND. – Brian Minton Nov 23 '15 at 14:50
  • Most SSDs use NAND, although better ones may use faster memory like DRAM.
  • I think one of the biggest differences is simply that SSD drives are made to a higher standard than USB flash drives. Flash drives are typically used for data transport and short term storage, so they don't need to be as reliable as an SSD.
  • 1
    DRAM is certainly faster than NAND, but is volatile even when powered and hence needs refreshing. – sawdust May 25 '15 at 5:06
  • @sawdust: Yes. I can't tell if you're just elaborating or if you disagree with something I said. (I only ask because someone downvoted this answer.) – pyrocrasty May 25 '15 at 14:37
  • Both. I downvoted your answer because DRAM is not used in SSDs for non-volatile storage as you wrote. – sawdust May 25 '15 at 18:28
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    I didn't claim they were used for nonvolatile storage. I said they were used in some more expensive SSDs, which is true. See here under 'ultrafast rackmount SSDs': "In today's market the fastest rackmount SSDs on the SAN are always RAM SSDs.". Another article here. – pyrocrasty May 25 '15 at 19:03
  • SSD in general mean any drive with no moving parts so the RAM is indeed SSD , but you can't compare RAM to flash drive so he definitely mean the non-volatile SSD and so your answer is wrong and i think you are confused between these two SSDs – robert Mar 27 '16 at 18:41

There are a few articles out there about the differences between SSDs and Flash drives.

  1. SSD just means a hard disk that doesn’t move
  2. Flash is a type of memory that is very fast and doesn’t require continuous power (non-volatile)
  3. SSDs used to use RAM, but now use Flash instead
  4. In short, you shouldn’t compare Flash to SSD just as you shouldn’t compare batteries to lithium-ion. In both cases the latter is a type of the former.

https://danielmiessler.com/blog/the-difference-between-ssd-and-flash-hard-drives/ http://www.mynetworks.me/2010/12/20/ssd-solid-state-drive-vs-flash-drive-usb-drive/

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    The points don't sound logically correct, though. For example, #1 is not a difference since hard drives use platters whereas SSDs use Flash memory, more of a digression to the difference between SSDs and HDDs. – oldmud0 May 25 '15 at 13:48
  • However, the second article is very useful. – oldmud0 May 25 '15 at 13:50
  • "...compare Flash to SSD ... latter is a type of the former". That's the wrong way around. You mean "SSD to Flash". – pyrocrasty May 25 '15 at 14:30
  • @pyrocrasty No, not really. At its core, SSD is a form of flash based storage device. Without the key component, there would not be any SSD to talk about. Just like with the Li-ion, which is a form of battery. So no, it's "latter is a type of the former". In other words, SSD is to flash based memory what Li-ion batteries are to... well, batteries. Don't confuse the generic term "flash drive" (as in USB flash drive) with "Flash" (flash memory). – Samir Jun 28 '15 at 20:22
  • @sammyg: Flash is not the only type of solid state memory. At the very least, memory must be non-volatile to be called flash memory. However, volatile solid-state memory (RAM) also exists (and is sometimes used in hard drives, for that matter). Any flash-based drive is an SSD, but not all SSDs are flash-based. – pyrocrasty Jun 29 '15 at 23:56

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