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Not only here, but also in other fora (plural of "forum"), there has been a bit of trouble with SD cards.

[Image: memory-card-speed-classes-960x640.jpg]
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As you can see above, there is two "Class types" which can mean totally different things. Class 3 is faster than class 10, while class 4 is slower. Just look for the "U" with a number inside, and you should be safe.

So, I'll just crunch some numbers to show that the SD card is one of the important parts of the tinkerboard, that buying a cheap one will gain you absolutely nothing, and that you should at least buy a class 10 card.

So, first off, why is the speed important?
If you bought a tinkerboard, you probably want to watch videos, mostly in the H.264 format. This format officially was planned to use streamrates from 1.5 MBit/second up to almost 30 MBit/second (and further, of course, since it's still undergoing development!). The reality looks different: A BluRay using H.264 may easily bust the 50 MBit/second.
The better it looks, the more power it uses.

Looking at a class 2, 4, 6, 8, 10 micro SD card, then each class represents the MByte/Second available.
Now, if you looked at it carefully, the one thing says "Megabit", the other thing "Megabyte", so let's do some math;
A Class 2 SD Card can deliver 2 Megabyte per second, that's 16 Megabit per second.
In other words, watching normal videos on such a cheap SD would work, but going full HD would be a disappointment - because there is at least 14 MBit/Second missing.

Also, buying something cheap from amazon doesn't do you any good. There is a lot of people out there busy printing a U 3 on a C 2.
And its hard for normal people to prove that. 
Buy a good micro SD card from a reliable source, I bought mine in a drugstore, because they were selling those especially for video cameras.

Also, there is more to it than just watching videos. Even just starting up the OS benefits from higher speeds.
For more advanced users, igorpec postet a performance chart:
https://forum.armbian.com/topic/954-sd-card-performance
Yes, anything below Class 10 is pointless ... but since the class is only about sequential speed, this information is not enough. Our type of usage is completely different than typical application SD card was designed for. Media storage.

Better rely on something like this - comprehensive SD card testings:
https://forum.armbian.com/topic/954-sd-card-performance

Good random read and write are features you are interested in. If they are good, sequential speed is usually good enough.
You definitely have a point there, but I didn't want to make it too complicated, and focused on the media - playing movies is the big plus of the tinkerboard after all.
I for my own part didn't even know what those numbers meant when I bought the tinkerboard, so I had to inform myself before buying an inexpensive, but useable, microSD.
And it seems to happen quite a lot that videos lag on the tinkerboard, without it being the tinkerboard's fault.

Thank you for the link, by the way, I'll consider it next time I upgrade my SD.
Also I think it would be good if people could write their experience with different MicroSD cards in here.
Quote:is the big plus

Until you click on playing the movie you rely on random read which is in most cases terrible. If you want to collect usable SD card comparison data, check my link again. That is the only proper technical way to compare. If you want to collect a quality/endurance data too, things become complex.
Bottom line. If you compare SD card speed only on sequential read/write, you usually fail. On most boards, SD card speed is anyway limited to a bus speed which is around 23MB/s.
Ooh! This Thread deserves to be made Sticky!
As the MicroSD Cards are rather Critical to the healthy operation of a Tinker Board, these references can be a lifesaver!

Thanks for the timely reference, and I hope others see it as useful as I do!
At least one happy customer Wink