Everything you need to know about Memory Cards that you didn’t and common mistakes people make

I’ve seen this happen over and over again: A new, simple-to-use technology arrives, with its own new acronym. Then someone improves upon it, extending the acronym. Then there’s another one. Soon you’ve got a market of confusing alphabet soup.

So let’s start with the basics. Secure Digital (SD) is the current standard for removable flash storage cards in mobile devices. They come in three physical sizes with numerous speeds and capacities.

A number of seldom-used features justify that S for Secure. One of them, Digital Rights Management (DRM), is for Hollywood’s benefit, not yours. Other, more consumer-friendly features include a write-protect notch (only on the full-sized cards) and password protection on a few select devices.

0123-regular-and-micro-sd-100224975-small.jpgAs I said, they come in three sizes:

  1. SD cards, the original form factor, measures 32x24mm. PCs often have full-sized SD Card slots.
  2. Mini SD cards measure only 21.5x20mm. These are now pretty much obsolete.
  3. Micro SD cards, which measure only 11x15mm, could be easily lost through a hole in your pocket. These little cards are very popular today, especially with Android devices.

Mini and Micro SD cards often come with adapters that allow you to plug them into a full-size SD slot.

In addition to their size, these cards are classified by standards that define their maximum capacities:

  • Standard Capacity (SDSC): 2GB maximum
  • High Capacity (SDHC): 32GB maximum
  • Extended (or eXtended) Capacity (SDXC): 2TB maximum, although as I write this, you can’t buy a card with a capacity over 128GB..

There’s another variant, SDIO (Input/output). These cards, which work only in very specific devices, offer more than simple storage. They can add GPS, Wi-Fi, and other features to your devices.

SD Cards are also classified by speed, marked by a letter C (for Speed Class) with a number inside, which represents megabytes per second (MB/s). These speeds are often described by how well they can write a video stream. The lowest rating, C2, can manage only standard definition. The fastest, C10, can manage full HD with room to spare.

Some high-end cards have a U instead of a C. These are Ultra High Speed (UHS) cards intended only for specific devices.

When shooting digital photographs, you can usually feel the difference between a cheap Class 4 SD card, with its minimum 4 MB/s write speeds, and a faster Class 10 or UHS (Ultra High Speed) card. SD speed classes can be really confusing–Class 10 cards are technically the fastest, with required 10 MB/s minimum read and write speeds, but there’s a world of difference between a basic Class 10 card and a Class 10 UHS card, which can operate at quadruple the standard SD clock speed. Slap one of those cards, like the SanDisk Extreme Pro, into a camera, and you’ll feel the difference–photos write to the card in a snap and reviewing a shot won’t leave you staring at a blinking LED for three seconds.

RECOMMENDED : All you should know about Samsung UFS Memory Card, 5X faster than SD Cards

The speed of the card makes a difference, but a fast card isn’t guaranteed to reach its potential in every device you use it in. Devices like digital cameras talk to SD cards with host controllers, and those host controllers can vary in speed and compatibility. For example, older host controllers only support the SD and SDHC formats, not the more recent SDXC. Using a really fast SD card with a slow host controller is a bit like plugging a USB 3.0 flash drive into a USB 1.0 port. You’re not going to come close to maxing out what the card is capable of.


Next time you buy an SD card–or anything that uses one–keep in mind that hardware like the memory controller and CPU, even in a brand new camera, may dramatically undercut what the card should be able to deliver.

Common mistakes people do when buying a memory card

Whether for a phone, a camera, or some other gadget, shopping for microSD cards seems like a pretty simple thing to do, right?

Yet there’s a lot more to them than you might realise, and it’s surprisingly easy to wind up falling into a number of traps: overpaying, terrible performance, or the card not working at all.



Part of the reason is the number of specs related to memory cards. So let’s take a look at how to read these specs, how to determine which one is right for you, and which mistakes to avoid.

1) Buying Incompatible Cards

When you talk about microSD cards, you are mostly talking about the form factor. All microSD cards fit into all microSD card slots, but they won’t all work. There are three different card formats, as well as different standards, that determine compatibility.

The three formats, which you’re probably already familiar with are SD, SDHC, and SDXC (or microSD, microSDHC, and microSDXC, but both micro and full-size cards are based on the same spec). Each format is defined in the SD specification, but they don’t work in the same ways.

As a result, the formats are not backwards compatible, and you cannot use newer cards in hardware that only supports older formats.



The differences between the three formats are significant:

  • microSD: Has a capacity up to 2 GB, and can be used in any microSD slot.
  • microSDHC: Has a capacity of more than 2 GB and up to 32 GB, and can be used in hardware that supports either SDHC and SDXC.
  • microSDXC: Has a capacity of more than 32GB and up to 2 TB (although at the time of writing this, 512 GB is the largest available card), and can only be used in devices that support SDXC.

In addition to checking that a card’s format is compatible with your hardware, you need to check a few other details, too.

Physical Size

First, hardware that supports microSDXC slots won’t automatically support every size of card in this format. The HTC One M9, for example, officially supports cards up to 128 GB and might not work with anything larger.



And if you’re planning to use your microSD card with your PC at any point — for example, to move files on and off — you also need to ensure your PC supports the file system that the card is formatted with. MicroSDXC cards use the exFAT system by default. Windows has supported it for over a decade, but OS X only since version 10.6.5 (Snow Leopard).

Ultra High Speed

The SDHC and SDXC formats can support the Ultra High Speed (UHS) bus interface — faster circuitry that enables data to move at a quicker rate. The two versions of UHS are UHS-I (with bus speeds of up to 104 MBps) and UHS-II (up to 312 MBps).

In order to benefit from the increased performance of UHS, it needs to be supported by your hardware. UHS memory cards will work in older slots but with a reduced bus speed of 25 MBps.

2) Choosing the Wrong Speed

Identifying the speed of a microSD card is just as complicated as deciphering formats and compatibility. There are four ways to show how fast a card is, and it’s not uncommon for manufacturers to use all of them.



Speed Class

The Speed Class shows the minimum write speed of a memory card in megabytes per second. There are four Speed Classes defined as follows:

  • Class 2: At least 2 MBps.
  • Class 4: At least 4 MBps.
  • Class 6: At least 6 MBps.
  • Class 10: At least 10 MBps.

Showing base level performance helps you to identify whether a card is suitable for a specific task, but because it makes no comment on maximum speeds, it’s technically possible for a Class 2 card to be faster than a Class 6 card. Class 10 cards should be noticeably faster, though, as they have a bus speed of 25 MBps (compared to 12.5 MBps on Class 2 to Class 6 cards).



UHS Speed Class

UHS Speed Class shows the minimum write speed for microSD cards that support the UHS-I and UHS-II bus speeds. We’re listing it as a separate category because some manufacturers list both classes on their cards. The two UHS Speed classes are:

  • U1: At least 10 MBps.
  • U3: At least 30 MBps.

Rated Speed

While it’s generally safe to assume that a higher Speed Class correlates to faster all-around performance, and UHS cards faster still, some manufacturers also quote a maximum speed for their products.

These speeds are shown in megabytes per second and help you pick out the absolute fastest cards. The speeds are based on manufacturer tests, however, so they may represent a best case scenario rather than real world performance.



In practice, there are other external factors that will affect read and write speeds. If you’re copying files to your PC, for instance, your PC’s specs — and even the USB cable you’re using — will play a role.

Relative Speed

The other way manufacturers show the speed of their cards is a throwback to the old CD writing days.

The original transfer rate for CDs was 150 KBps. As drives developed, they would advertise themselves as being 2x, 4x, 16x, and so on, showing how many times faster than 150 KBps they were.

MicroSD cards are also often labelled in this way. When a card is described as 100x, it means 100 x 150 KBps, which is 15 MBps. That speed is, again, under ideal lab conditions.

3) Wrong Cards For the Task

When buying a microSD card, it’s important to pick one that is right for its intended use. This means finding a card that is large enough and fast enough — not necessarily the largest and fastest card out there. High capacity UHS-II U3 cards often still have a price premium and you won’t always notice the benefits they offer.

If you’re using a microSD card to increase the storage on your smartphone, then it makes sense to pick the highest capacity you can. At the same time, speed won’t be a major priority since you won’t be transferring large files back and forth frequently.

The exception would be if you’re using a newer phone that can shoot 4K video. Just as with a 4K video camera (like the GoPro), large capacity and high speed are musts.



Panasonic recommends UHS Speed Class 3 (U3) for shooting 4K video. For full HD video, it suggests Class 10 or Class 6 at a push. If your card’s write speed is too slow, it will result in dropped frames and produce stuttering video.

For photography, some users prefer several smaller cards to a single large one so they minimize the risk of losing all of their photos if a card corrupts. If you’re shooting RAW, where files might be 20 MB or more, you’ll benefit from having U1 or U3 speeds (but they require at least SDHC format).



And in case you’re wondering, there’s no difference between a full-size SD card and a microSD card in an SD adapter. If your camera only has an SD slot, you can still use a microSD card in it.

4) Buying Fake Cards

It sounds like an obvious thing to avoid, but sadly, buying fake memory cards is incredibly easy.

If you find a good deal on branded memory cards from a non-reputable seller, then there is a real risk that it may be counterfeit. In fact, a few years ago a SanDisk engineer reportedly stated that as a many as a third of all SanDisk-branded cards were fakes. It’s unlikely that that number has declined since.

The buying guides on Ebay include a page on spotting counterfeits due to how common they are. Amazon Warehouse sellers have been accused, too. If you’re buying from a source you’re unsure about, check the reviews first.

Counterfeit cards are configured to report the capacity that is printed on the packaging, but actually contain far less. You won’t notice this until the card fills up unexpectedly quickly.

Use the utility H2testw for Windows, or F3 for Mac and Linux, to check that the cards you already own are genuine.

5) Cheaping Out on Brands

We’ve all owned flash memory cards that have stopped working for no apparent reason. While reliability is generally excellent, microSD cards do fail, and when they do, they’ll take all your data with them.

For this reason, buying cards from big brands is always better than buying no-name cards for cheaper. You can expect better performance, greater levels of reliability, as well as more robustness, with cards routinely protected against shock, water, and even airport X-rays.

You also get things like a lifetime warranty (where your card will be replaced if it fails) and access to image recovery software (to retrieve data from a damaged or corrupted card). Manufacturers such as Lexar and SanDisk offer this as standard.

So that’s probably all folks. There’s what you need to know about sd cards and hopefully now you can make an educated choice.

What do you look for when buying microSD cards? Have you ever encountered compatibility problems or fakes? Let us know in the comments.

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