hutskiey

I was shopping online for some new microSD cards and got thoroughly confused by the different classes/speeds. Reading an article by the organisation that sets and regulates SD card performance standards (Greater Performance Choice) made me even more confused.
We use microSD cards in compact cameras, Samsung and Nokia Lumia smartphones, and for storing and saving games in our original Wii. Does it matter which class of card is used?
Mark Campbell
Secure digital (SD) cards started out as a wonderfully simple Flash storage format, and quickly replaced CompactFlash, MMC and other types of card. But as usage grew, people began to need different speeds and sizes. The result is the chaos we enjoy today.

For standard applications like single-shot compact cameras, it doesn't matter which class of SD card you buy, as long as it's compatible. Storing a 500K to 2MB file is not hard. However, if you want to shoot a rapid sequence of photos, the card has to be fast enough to keep up. If you want to shoot high-definition videos, keeping up can be a challenge. For this sort of application, you'll probably need a fast card.

When buying an SD card, you have to consider three things: the physical size, the storage capacity, and the speed at which it can write data.

Physical size

There are now three sizes of SD card. The original format started with cards measuring 32 x 24mm, which was very small for the time. These are still common in digital cameras, audio recorders and similar products. Smartphone makers wanted smaller cards. This resulted in the miniSD format, measuring 21.5 x 20mm, and then microSD cards, measuring 11 x 15mm.

You can use a miniSD or microSD card in an SD card slot by plugging it into an SD-sized adapter, and some mini/micro cards are sold with them. Obviously, you can't fit an SD card into a miniSD or microSD slot.

I assumed most of us needed different sizes, so it's handy if all your devices use the microSD format. However, other things being equal, full-size SD cards are generally faster than small ones.

Data storage

When it comes to storage, there are three different types of card: SD, SDHC (High Capacity) and SDXC (eXtended Capacity).

Originally, an SD card could hold up to 2GB of data using Microsoft's FAT-12 or FAT-16 file format. This was a huge amount in 1999, and most cards stored a lot less. In 2006, when the 2GB limit was becoming a problem, the industry introduced high capacity SDHC cards that could store up to 32GB using FAT-32. After that, in 2009, came SDXC cards able to store up to 2TB using Microsoft's proprietary exFAT (Extended File Allocation Table) format.

Apple started supporting exFAT with Mac OS X 10.6.5 (Snow Leopard), which makes exFAT the best choice for shared hard drives or transferring very large data files. It also means that future SD cards will be able to use exFAT up to 512TB and, in theory, beyond. Either way, the SD Card Association provides a free SD Formatter 4.0 for SD/SDHC/SDXC program for Windows and Mac OS X for people who want to check or reformat cards. This optimises performance and protects the Secure Digital card's secure 'Protected Area'.

Today, 8GB and 16GB cards are big enough for most purposes, though people may well choose a 32GB or 64GB card to expand the storage in an MP3 player or mobile phone. If buying a 64GB or 128GB card, it's very important to check that your device can support it. The Samsung Galaxy S4 can handle 64GB cards, but most phones can't, and I don't know of a European or American phone that supports 128GB cards. If you need 1TB or 2TB of storage, you'll have to buy an external hard drive.

Card speeds

Things get confusing when it comes to speed ratings. The SD Card Association has different classes of cards, and produced a bunch of mandatory logos with the class number inside a big C. However, Class 2 is 2MBps, Class 4 is 4MBps, Class 6 is 6MBps, and Class 10 is 10MBps or faster -- sometimes much faster. For this reason, faster cards are often marked with the speed in large letters, such as 45MB/s or 90MB/s.

If you check the specifications of your devices, you should be able to find out how fast it can write data to an SD card. As long as your cards can handle that data rate, you should be OK.

Note that the class rating shows a minimum speed, not the actual speed. A good Class 2 card may work faster than a Class 6 or even a Class 10 card. The only way to find the real performance of a card is to run a benchmark test.

Also bear in mind that megabytes per second -- MBps, usually written as MB/s on cards -- are eight times faster than megabits per second (Mbps), because there are 8 bits in a byte. A 2MBps card is a 16Mbps card.

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Finally, SD card speeds are sometimes cited as '100x Speed' or whatever. This is somewhat akin to measuring things in football fields or areas the size of Wales. It's based on the speed of an original CD-Rom drive: 150 KBps. You will recall that you ended up with a 16x or 48x CD-Rom drive, and you can extend the idea to 48x or 400x SD cards. I don't find this a particularly useful way of rating SD cards, unless your camera manufacturer specifies 50x or 100x cards, or whatever.

Bus speeds

Unfortunately, it takes two to tango. The speed of the card is one factor. The other is the speed at which a particular device can send data to the card. This depends on the speed of the input/output bus that connects them.

For the first types of card, everybody used a standard bus, so nobody had to worry about it. When speeds reached 10MBps and above (Class 10), we needed a faster bus, so the High Speed Bus was introduced. After that came the Ultra High Speed Bus UHS-I and the even faster UHS-II, which guarantees a minimum data rate of 30MBps. These also have their own logos, with cards being marked U1 or U3.

As mentioned, some SD cards support speeds of 95MBps or more, but very few applications need such fast cards. A 2MBps card should be able to record standard video, a 4MBps card should record HD (ie 720p) video, and a 6MBps card should handle Full HD (ie 1080p) video, depending on the camera or camcorder.

Buying cards

For most purposes, you can buy any card that your device supports, and ignore the speed ratings. For example, your camera's spec will say that it takes microSD, microSDHC or microSDXC cards, so just buy whichever type it says. Other things being similar or equal, buy whichever card is faster.

Not many devices have UHS-I buses at the moment, and usually they are more advanced models. For example, looking at Canon DSLRs, the EOS 500D takes SDHC cards, the 600D can use SDXC cards, and the 650D is the first model that can handle UHS-1 or U1 cards. (The EOS M also sports UHS-1, as do the Nikon D5100 and D7000.)

In general, I'd avoid buying UHS-1 cards unless your device specifies them, because the new bus has new pins and reassigns some old pins. Cards with both logos – ie C10 and U1 – may work in most (but not all) other devices, but will usually cost more. They still won't write data faster than your camera can send it.

You could argue that UHS-1 will become common and that people will reap the benefit when they buy new cameras, or whatever. That might be true for people who are planning to buy more advanced devices fairly soon. The rest of us can wait until UHS buses filter down to cheaper devices, by which time, UHS cards will be cheaper.

I'm not against spending money on SD cards, but I think it's better to spend it on respected name-brand cards and not on cheap ones. As was the case with CD-R discs, not all Flash memory chips are created equal. Personally, I use SanDisk and Samsung cards, but Toshiba, Panasonic, and Lexar Pro and Platinum II cards also have good reputations. Panasonic invented the SD format, and developed it with SanDisk and Toshiba.

The whole ‘does an Android smartphone need expandable storage’ debate has been raging for many years and it has been a real roller-coaster of a ride. One year Samsung’s flagships have microSD card slots, the next year they don’t, then support is back again. Up and down, up and down. Google never seems to want to include microSD card support in its Nexus line, but the individual OEMs who manufacture the Nexus devices do include support on other handsets they make!

If you do have a smartphone that does include a microSD card then one of the first questions you are likely to ask is this: What is the highest capacity microSD card that I can use on my phone? And it is a good question, but the answer may not be as simple as you were hoping. To get to the bottom of this we are going to need to look at the different microSD standards, the different file systems supported by Android and by desktop OSes like Windows & OS X, plus we will need to take a peek into the murky world of patents.

microSD cards and the SD Association

Let’s start with the basics. The standards for SD cards and microSD cards is defined by the SD Association. It was set up in 2000 by Panasonic, SanDisk and Toshiba to develop and promote memory card storage standards. Basically the SD Association makes sure all SD related technology (readers, cards etc) are compatible. At present there are three standards when it comes to the capacity of SD and microSD cards:

Type of card	Max Capacity	File System	Backwards Compatibility
SD	2GB	FAT32	SD
SDHC	32GB	FAT32	SD, SDHC
SDXC	2TB	exFAT	SD, SDHC, SDXC

So basically the original SD card standard supported cards up to 2GB. Then came SDHC which extended the capacity to 32GB and then more recently the SDXC standard was released to boost the capacity to 2TB. All the standards are backwards compatible, which means that a device with a SDXC support can use all three types of SD card, but a device with a SDHC support can only read SDHC and SD, but not SDXC.

When it comes to handsets you may have noticed that some OEMs will says something like this, “expandable storage via microSD card up to 32GB.” what that probably means is that device has a SDHC compatible card reader. Technically any Android device which supports SDXC could advertise “expandable storage via microSD card up to 2TB.” But because 2TB microSD cards don’t exist (yet), most OEMs will say something like “expandable storage via microSD card up to 128GB” where the “128GB” bit could be a different capacity depending on when the device was released and what is the highest current microSD capacity that is commercially available.

FAT32, exFAT and Microsoft

As well as defining the physical characteristics of the memory cards, the SD Association also recommends how data is stored on the cards. If you imagine that the SD card is a block of storage space, any device that wants to read a file from that block needs to know where the file starts on the block and where it ends. It needs to be able to find that data from the file name (actually the full path name) and it also needs to know some information about the file’s permissions, etc. The way the files are organized on a storage device is controlled by the file system. There are lots of different file systems. On Windows you are probably using NTFS, on OS X it is HFS+ and on Linux most likely ext4.

Back in the late 1970s Microsoft produced its first version of a file system called FAT (File Allocation Table). It was originally developed for use on floppy disks, however over the years it has found its way onto hard disks, DVDs, USB flash drives and SD cards. It was the default file system for Windows until Windows XP. There have been several different variations of FAT (mainly based around the size of the table elements in the allocation table). These different variations are known by the number of bits that can be stored in each table location. The original FAT used 8 bit entries, and is today referred to as FAT8, then came FAT12, and with the inclusion of a hard disk in the IBM PC AT we got FAT16. For Windows 95 OSR2 Microsoft released FAT32.

The Extended File Allocation Table (exFAT) file system is another Microsoft design.

As you can see from the table above, FAT32 is the recommended file system for SD and SDHC cards. However FAT32 does have some limitations including a maximum file size of 4GB. While the idea of a 4GB file was probably unimaginable to people installing Windows 95 (from floppy or 650MB CDROM), today recording high quality video can easily create a 4GB file. To overcome these limitations a new filesystem was adopted, exFAT.

The Extended File Allocation Table (exFAT) file system is another Microsoft design, that was first introduced in 2006 as part of Windows CE 6.0. It allows for files that are larger that 4GB and it was adopted by the SD Card Association as the default file system for SDXC cards. For the testing section below I bought a 128GB microSD card from Kingston, and by default it was formatted using exFAT.

Since FAT32 and exFAT belong to Microsoft here we actually find how Microsoft is managing to make billions of dollars from Android. If an OEM wants to use FAT32 or exFAT it needs to pay a license fee to Microsoft. I am not one for conspiracy theories, but it is “interesting” how the SD Association used exFAT for SDXC. FAT32 is possibly understandable, it was the dominant industry standard, but exFAT was not used by anyone other than Microsoft, then all of a sudden every smartphone OEM, digital camera maker, media player manufacturer might need to pay Microsoft a royalty to support SDXC and exFAT… hmmm…

Interestingly Windows won’t format SD cards bigger than 32GB using FAT32. However it is possible using third party tools. If you try to format a 64GB (or larger) USB flash drive or SD card under Windows you will have to choose between NTFS and exFAT.

Adoptable storage

Since we are talking about microSD cards, it is worth mentioning adoptable storage. Once a microSD has been inserted into a smartphone, the question arises, how should Android use it? The simplest way is for the extra storage to be used for media like photos, music or videos, and treated in a similar way to a USB flash drive on Windows. The phone isn’t dependent on the card in anyway and can operate with or without the card. This allows the user the freedom of taking out the card and using it on a PC, and then popping it back into the phone when needed.

However it would also be nice to have the option to use the extra storage as if it was internal storage and install apps on to it, plus store app data on it. This has been possible in the past with the various “move to SD” mechanisms, however it has one major pitfall, security. If I move an app over to the SD card and start storing my personal private data on that card then I open myself up to data theft. If someone removes the SD card from your smartphone they only need to plug the card into a SD card reader on a PC or laptop to get access to your unencrypted data.

Android 6.0 Marshmallow introduced the idea of adopting external storage so that it acts like internal storage. When a microSD card is adopted, it is formatted and encrypted to only work with that device. Now you can safely store both apps and private data on the card. One interesting feature of adoptable storage is that it isn’t limited to 2TB like SDXC, but can actually use media up to 9 Zettabytes… Now, where did I put that 9 Zettabyte microSD card, I know it is here somewhere!!!

Flash drives and USB OTG

Although we have been talking about SD cards, it is interesting to note that much of our discussion also applies to USB flash drives. Many Android devices can connect to USB flash drives via a microUSB to USB OTG adapter. Like SD cards, USB flash drives can be formatted as either (but not limited to) FAT32 or as exFAT. Also the restrictions about file size etc apply equally to FAT32 formatted USB flash drives.

As I mentioned earlier, Windows won’t format large USB drives as FAT32, you need to pick exFAT, rather than NTFS, if you want to have any chance of the drive working with Android. Having said all that, my 128GB USB flash drive (from Lexar) came pre-formatted as FAT32, which means it wasn’t formatted using the built-in Windows format tool!

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Testing some devices

To test the support for FAT32, exFAT, and SDXC I got hold of a 128GB microSD card and a 128GB USB flash drive. Then I tried to use them on a variety of different devices from the Raspberry Pi to a Sony TV along with lots of Android devices. This is what I found out:

USB flash drive formatted exFAT

For this test and the next one, I took my 128GB USB flash drive, copied some files onto it and connected it to a selection of devices, using a OTG adapter when necessary (i.e. for the Android phones).

Let’s start with what didn’t work. The Raspberry Pi running Linux won’t read exFAT files and neither does a laptop running Linux. This is due to the licensing issues around exFAT, it belongs to Microsoft and while there are some open source exFAT drivers they aren’t in the mainstream for legal reasons. However the exFAT formatted USB drive is recognized by Chrome OS running on my ARM based Samsung Chromebook. As you would expect Google and Microsoft have a wide ranging set of patent and cross licensing deals (which probably cover FAT32 and exFAT). They even recently agreed to stop complaining to the regulators about each other.

There were two other devices which I tried which didn’t work with exFAT. One was my Sony Bravia (non-Android) TV and the other was a Motorola Moto G (2015) running CM 12. All the other devices I tested work fine including the Samsung Galaxy S7, Kindle Fire, Samsung Galaxy Note Edge (AKA Note 4 Edge), Asus Zenfone 2, OPPO F1 Plus, and Huawei Mate 8.

USB flash drive formatted FAT32

I reformatted the USB drive as FAT32 (using a third party tool, as Windows won’t do it) and tried it again on the devices that had problems with exFAT. The good news is that the Raspberry Pi and my laptop running Ubuntu were able to read the USB drive without any problem. Which is to be expected really. Also my Sony TV had no trouble with the FAT32 formatted USB drive. I did a quick couple of tests to make sure that some of the Android devices could still read the flash drive using an OTG cable, and they could. The only device that still didn’t want to read the drive was the Moto G running CM 12.

microSD card formatted exFAT

For the next two tests I used a 128GB SDXC microSD card. For the first test it was formatted as exFAT. I copied over some files and then tested the card in a range of different devices. Starting with what didn’t work, the SD card wasn’t recognized by the Xiaomi RedMi Note 2, the ZTE Star 2 nor the Elephone P6000. The latter two are running Android 4.4. KitKat.

However the card worked perfectly on a bunch of other Android devices including the Huawei P9, the Samsung Galaxy S7, the Huawei Mate 8, the Galaxy Note Edge, the Moto G (2015) running CM 12, the ASUS Zenfone 2, the OPPO F1 Plus, Samsung Galaxy S3 Neo, and my Samsung Chromebook.

microSD card formatted FAT32

I changed the format of the microSD to FAT32 and tried the devices that didn’t previously recognize the card and the good news is that they worked! The Xiaomi RedMi Note 2, ZTE Star 2, and the Elephone P6000 all mounted the card and where able to read the files on it. As a side test, I reformatted the card again as exFAT an put it back into the Xiaomi RedMi Note 2. As before the card wasn’t recognized, however there was a option to reformat it. When I did the RedMi Note 2 reformatted it as FAT32 and it worked!

Rssb beas india soul. I tried the FAT32 formatted card on a Raspberry Pi 3. The Pi was able to boot and install Raspbian (via NOOBS) from the card without any problems.

Wrap-up

So what does all this mean? Basically it seems that support for exFAT is the stumbling block for some devices. For a device to officially support large SDXC cards it must be capable of reading and writing to exFAT formatted media. During my tests I found several devices that don’t support exFAT and so don’t officially support SDXC cards over 32GB. However in every case where a device couldn’t access the 128GB card with exFAT, I was able to reformat the card as FAT32 and it worked, even in phones that were 2 years old and running Android 4.4 KitKat.

Bottom line, if you have a microSD card slot in your phone it will probably work with large (>32GB) SDXC cards and if it doesn’t then a quick reformat of the card to FAT32 will likely solve your problems. As for 2TB cards, when they do eventually come out, your phone should support those as well!

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