SD Card vs. SSD: What Is Really the Difference?

The solid state drive has given us a leap forward in the ability to open applications and read files quickly. Its storage mechanism operates under the same principles that other flash storage mediums use, namely non-volatile memory, which prevents memory from disappearing due to loss of power like it does in RAM. Since both SD cards and SSDs use solid-state storage and have no moving parts, is there any notable difference between the two types of memory? Shouldn’t a massive-capacity SD card be just about the same thing as a small SSD?

sdnand-memorychips

NAND flash memory depends on other hardware installed on the device or embedded into the chips. A NAND cell is basically a series of semiconductors that hold data inside them. The speed at which they read and write information is almost entirely contingent on how they are arranged and how the controllers that pick up and send the data coordinate the process.

Theoretically, you could end up with an SD card that writes and reads just as fast as an SSD. Most of the time an average card will not be that fast. The limited amount of space they provide doesn’t allow manufacturers much room to maneuver and creates an environment where massive amounts of data can be moved in tiny clock cycles.

This is not to say that there aren’t different types of NAND flash memory, each with their own disadvantages and advantages. However, you could theoretically move NAND from an SSD (such as the 3D TLC NAND found in the Samsung SSD 850 EVO) into an SD card. For the SD format to work it just needs to be capable of communicating with the devices that read it.

Since SD cards have a limited amount of space, the microcontroller that fetches storage and writes to it is usually pushed to the edge of the card, like the following image.

sdnand-chips

There are only so many instructions that could be programmed into a microcontroller of that size, and with such a tiny infrastructure, the way an SD card handles data is rather rudimentary. It will have a tendency to store data wherever there is free space and read things in as orderly a manner as possible.

This isn’t true of SSDs, which have the luxury of fitting all of their memory and their entire infrastructure into a space that fits into the average computer’s drive bay. Here’s what that looks like. (The controller is circled just like in the image above.)

sdnand-ssd

The entire infrastructure of the SSD is built to ensure that no single cell is used more than the others, keeping every file operation as balanced as possible, which is what you’d expect from a drive that does a lot of read/write operations.

The larger amount of space also allows manufacturers to insert chips that store cached data, which is crucial for managing heavy and repetitive operations quickly. No time is wasted and everything transfers fluidly.

In addition to this, the added bulk of the drive allows it to dissipate more heat. This makes it capable of having more power-hungry controllers that would be unfeasible in an SD format (because it both draws more power than small handheld devices could provide and heats up significantly).

The gist of all of this is that each platform was designed to work in specific environments. SD cards are best used for storing files and playing them back, while SSDs are optimized for running the operating system partition of a computer and everything such a task would demand. One has a simpler role while the other needs to be smarter and more adaptable. It’s not just a question of speed here but also one about workflow and versatility.

What do you think about the prospect of having SD cards with sufficiently complex microcontrollers that allow them to work as swappable drives for phones where entire applications could be installed on them instead of needing to have part of the core app in internal NOR storage? Tell us what you think in a comment!

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