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Update 03/04/2015: Added info on WiFi capable SD cards and filesystems.

Before you wonder what this might be about I will tell you ;)

We now have so called UHS cards or SDXC cards as they are also named.

It is the next big thing after SDHC cards and gives us now three main classes in terms of size and speed for SD card.

1. SD card - up to 2GB although versions with 4GB do exists for old cameras.
SD cards are known to be quite slow in terms of write and read speeds.
They are also not that sommon anymore.
Maximum data transfer speed: 25mb/s - theoretical, praktical they get to around 10mb/s write speed and about 20mb/s read speed for good cards.
Most SD cards however will never reach those speeds as only quite slow memory was avialble at those times and high speed chips would have made the cards too costly.

2. SDHC card - from 4 to 32GB in size and available in three main speed classes.
Max. possible transfer speeds:
Class 2 - 2mb/s
Class 4 - 4mb/s
Class 6 - 6mb/s
Class 8 - 8mb/s
Class 10 - 10mb/s
Their main advantage is not just bigger size but much higher transfer speeds, which was welcomed by camera manufactures.
Most current devices are compatible with SDHC cards, although some older devices might only accept a size of 8GB due to restrictions in the filesystem used.

3. UHS or SDXC card - up to 2TB or 2048Gb, the latest technolgy for SD cards.
SDXC cards are a breakthrough in size and speed:
Max. possible transfer speeds:
UHS - 1 : 10mb/s
UHS - 2 : 20mb/s - not very common type though so beware when you see them as it might be a fake.
UHS - 3 : 30mb/s
However these standards were defined 2009 by the SD association and we now have memory chips available that can perform at speeds of up to 90mb/s.
The actual UHS rating is therefore the minimum the card must be capable of, meaning a UHS - 1 card can be faster than the UHS - 3 rating in real life.
Biggest problem with UHS or SDXC is that only the card is backwards compatible but not the transfer speed.
An older card reader might be able to read and write a SDXC card but only a the max SD class rating it can handle, usually around the 7-10mb/s mark for writing to the card.
Some device simply refuse to recognise a SDXC at all and will prompt you to format the card or that there is no card.
In such a case do not format the card - check it in a proper reader on your computer!

Step 1: Other Types of Cards That Look Quite Similar

There are many types of cards that look quite similar to a SD card.
To name a few: MMC, Memory stick, xD, XQD and of course CF cards.

Most of them had been invented along the SD card but never made a big breakthrough as they were/are limited to devices needing them.
The big exception is the CF card, which is mostly used in digital cameras but also GPS devices and as replacement for old hard drives. CF card can offer quite extreme speeds compared to SD / SDHC cards.
They all have been designed to meet the needs for higher speeds and more storage capacity, most offered much higher capacities as SD cards which made them the perfect alternative at that time.

As a special case you can also find WiFi capable SD cards.
These give you the option to upload and share your images using a connected phone, tablet or computer.
They are available from all good name brands but stay aways from Transcend as as it seems they messed things up with the GNU licensing.
You pay quite a bit more for a card of similar size to get WiFi and not always it is clear to see what real speeds the cards are capable of.
So as with other things do your homework first in you need a very fast SD with added WiFi functions.

For more details please refer to Wikipedia.

Step 2: Can I Use UHS Cards in My Device or Card Reader??

In most cases the answer is yes, although some devices won't be able to recognise UHS cards due to hardware or driver limitations.
It is always advised to check for compatibility before buying!

The main problem and reason for complaints however is not being able to use the new card, but to actually use the advertised speed.
As mentioned in the previous part UHS is not downwards compatible, at least never for the full speed.
Many user get their new card and start complaining or demanding refunds because it is actually slower than their old 8Gb SDHC Class 10 card.

Why is this happening?
Quite simply put, the technolgy of the the card has outrun the hardware trying to use it.
It is like getting a SATA controller for you old PC only to find out the new hard drive is not really fast at all.
The bottleneck in both cases is the hardware reading/writing the data and the technology used to transfer that data to the host.
For cameras and other devices you can only confirm with the manufacturer if you can fully utilize UHS cards.
For your computer and other devices using a USB card reader I will try to explain the "bottleneck" a bit further:

USB 1.0 should not be the fastest port you have, otherwise it is time to upgrade as USB 1.0 only offers a max transfer speed of 1.5mb/s.

USB 2.0 offers speeds of up to 35mb/s in a perfect enviroment but more like 20mb/s in real life.

USB 3.0 offers speeds of up to 500mb/s in a perfect world and 350-400mb/s in our real world.

So if you reflect on the previous data on UHS transfer rates it becomes clear why on a USB 2.0 port a high speed card will be stopped in it's tracks.

On top of these limits by design specs you also have the problem of your chipset and card reader to consider.
The chipset can affect the transfer speeds as it has to negotiate all data traffic on USB channels.
The more devices you have connected and are in use the slower transfer rates can get.
This can be hard drives, virtual reality goggles or WiFi adapters in particular when downloading at high speeds.
Wireless receivers for keyboard, mouse and similar are problem, same for printers.
The card reader is what kills most of our speed effords.
Keep in mind that it is the door between the fast data bus on the SD card and your USB port!
So a USB 2 card reader won't be able to give you full speed for your UHS card, not even on a USB 3 port.

Step 3: A Word on Card Readers

You made it this far so it is time to shed some light on card readers.

We now know that without a proper one we can't utilise the full potential of UHS cards, in some cases not even of SDHC cards.
This leaves the question how to find out what your existing reader can do and how to find a proper UHS capable card reader.

Before asian manufacturers flooded the market the type of connector was a good indicator of a good reader.
USB 3 uses blue plastic inside the USB plug.
USB 1 and 2 only use 4 connectors inside the plung, USB 3 uses 9, five in the front plus 4 more in the back, please compare the images:





But as soon as UHS card became more affordable we were flooded by cheap card readers claiming to be of high speed, UHS ready or USB 3 compatible.
Now they don't even bother and simply sell them without any useful information, but at least the misleading info is mostly gone.
Unless you have a really trusty seller your only good option is to buy brand quality readers!
It does not matter if it is Sandisk, AData, Panasonic any other top brand, they all offer proper readers and clearly state their capabilities on the corresponding website for the product.
Samsung went even further with their EVO range of cards and icluded a proper card reader with reatil packages meant for smartphone users.
Some even offer OTG or combo readers that can be used on devices that offer USB host capabilities - eliminating the need for an OTG cable and bulky card readers.
Without any desire to advertise I would like to point out a really good OTG reader that is capable of delivering top speeds together with a really tiny footprint: The Meenova "Dash Micro" - one of the smallest readers currently available.
But beware of copy cats!! A lot of asian online sellers and even retail shops sell very similar looking readers, some in fancy colors or with an additional slot to carry a spare card.
These copies are small but in most case unable to perform and in some cases won't even recognise all card types.

Step 4: Why Are There So Many Different Speeds When It Comes to UHS Cards?

This one is not so easy to explain without some very technical explanations...

When it comes to "speed" for a SD card it often comes down to what the user means by slow or fast and how he uses the card.
There is a UHS card for every purpose if you know what to look for!!

Ok, what is "speed" here?

We have different types of data transfers and how the memory is used.
Cameras for example need memory that can be written at super speeds to allow for fast burst shots or the recording of high definition video without compression.
As extended storage or for backup purposes you want fast reading and writing of big file sizes.
A smartphone or tablet user needs a card that is really good at the random read access of files of all sizes - after all the OS is constantly active on the card.

In technical terms we speak of:
Sequential read and write - the speeds for accessing single files, usually of bigger size like photos or movies.
Random read and write - the speeds for accessing data in various locations on the SD. An example would be booting an operating system.

The sequential speed is what most people are interested in when it comes to copying larger files.
In most cases it is the only speed definition that is important.
Depending if read or write speed is the main concern you can get optimised cards for the job.

The random speed is of great interest for everyone using the card in a phone or tablet for other than pure storage needs.
On phones the so called "external SD card" is not just storage but fully integrated into the operating system.
This means even the fastest cards for sequential access will be outperformed by a good card for random access.
A prime example for such a card is the Samsung EVO series.
The have been optimised for this type of use.
Most users won't know what they are missing until they actually try a card optimised for the job.

Transfer speeds in excess of 90mb/s are possible and we see even faster cards in the near fututre.
The question you have ask is if you really need to replace you old card for a new high speed model.
There is no point in getting a card if you can't use the full potential ;)
But if you decide to upgrade do some reseach first, check for benchmark tests on SDXC cards and select a card that will fit your needs not only in terms of storage size.
Especially users of digital cams should consider checking the compatibility first and if UHS is a valid option go for cards with really fast sequential WRITE speeds!

Step 5: File Systems and Formatting

In my other Instructable I already mentioned the SD formatter by sdcard.org.
This is the best tool for your SD cards as it will use all the correct settings to create a fast file system on your SD.

But with 256Gb or more you might be tempted to change the filesystem for compatibility reasons with a specific device.
As a rule of thumb cards up 32GB use the FAT32 system by default and cards of greater capacity usually come formatted in exFat.
Exfat also gives the best results in terms of speed for your SD card as it optimised for flash memory instead of a hard drive.

Partition tools can be used to create multiple filesystems on the SD, Fat32, NTFS, exFat, ext2, ext3, ext4 are the most common.
Windows however will only see the first partition on a SD card.
When formatting without using the SD formatter but the Windows Explorer instead (to change the filesystem) you should leave the cluster sizes set to what Windows suggest.
Only if the target device requires special settings it should be changed.
SD cards for Android devices for example can be created with a FAT32 partition (active) first and an ext4 partition in the second part of the memory.
Please be aware that formatting should be avoided.
A quick format is less harmful but a full format with erase causes a lot of stress and heat and can shorten the lifespan of your SD if done often enough.

<p>Super information for me. I'm starting to work with goPro cameras and pi.</p><p>Had no idea of what is available. You must be intimately involved in the trade ?</p>
Not at all.<br>Just collected some SD cards over the years and thought it might be good to share what I found.<br>These days there is everything in Google and other search engines, but finding what really is important can be tricky.<br>My Instructable is not what I call perfect or fully complete but gives a good start until the technology changes again.
what!? no eye fi?
<p>I did not include WiFi SD cards as they are still quite uncommon and more and more new cams offer WiFi for printing and other tasks now.<br>But I add it for you ;)</p>

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