eelnoraa
Contributor
This is older stock of Sandisk Ultra SD card. They aren't label C10 because they are C6. It use 24nm 3bit/cell memory. Which beside the speed, it is arguably a more reliable card.
This is UHS-I, which automatically means C10 at least. It is indeed a C10 card. It is 19nm 3bit/cell memory. It is faster mainly because it uses an more advanced ECC engine, so when write to the card, it doesnt need to be as precise as the previous model, and therefore can write faster.
---------- Post added May 5th, 2013 at 05:23 PM ----------
There are a lot of confusion about what speed class means. The truth is speed class is NOT sequential write speed, but rather random write of access unit (AU) of predetermined size. Within the AU, the write is sequential. The write speed then is measurement of the average write speed per AU. Speed class garantee this write speed per AU as a whole, not necessary the instantaneous speed of any moment.
The max AU size allowed is 4MByte for SDHC and 16MB for SDHC. Since the bigger the size, the faster write speed it will, most manufacture use these number as their AU size.
Take the older
64GB SDXC cards here. Class6 means for any random 16MB AU write, the average speed across any AU needs to be at least 6MB/s. At any moment, the write speed can drop below 6MB/s, but as long as at the end of the 16MB AU, it meets 6MB/s, it is still a C6 card.
Knowing the concept of speed class now, it is very important that the devices using the SD card need to follow speed class rules. If a device decides to write random data chunk of 2MB, then it should not expect the C6 card will delivery 6MB/s throuhout the write. Nowaday, most bigger device manufacture understand and follow the speed class rule, but I have some that don't.
Now, how is speed class related to sequntial write speed? Simply put, in order to meet class 10, the sequential write will have to be at least 10MB/s. But sequential write of 10MB/s doesn't mean it will meet class 10. In fact, one can easier design (poorly) a SD with 40+MB/s sequential write and yet fail Class10 requirement. Some well design SD card, like SanDisk ExtremePro, it will does 90MB/s sequenital while meeting class 10.
One more catch in Speed class. It is only garanteed when the card is in healthy state and after format. Healthy state mean it is relatively fresh, not a card you have cycled throught hundreds times. For the after format part, technically, it doesn't have to be after format, but it shouldn't already have random fragement of data in the card already.
