[HTC 10 Questions & Answers] lagittaja:
So I was anxiously waiting for HTC's reply about the UHS-I vs UHS-II.

Randomly looked at the SD820 specs five minutes ago. I'm feeling so dumb right now.

https://www.qualcomm.com/products/snapdragon/processors/820

Well there's our answer.

And since the A9 supported UHS-I U3, the 10 will also support UHS-I U3.

It will NOT "fully" support UHS-II U3 cards, they will work (backwards compatibility) but you will not get the full speed benefit of them!

Maybe we'll get SD 4.0 (UHS-II) with Snapdragon 830 or whatever it will be called..

I'm going to assume you mean with Adoptable Storage "enabled".

Well, of course an UHS-I U3 card. Sure you can buy and use an UHS-

II

U3 card but there is no real benefit to that because the 10 fully supports only UHS-I U3. Also UHS-II U3 cards are really expensive..

For user experience, buy the microSD card with the fastest

random 4K read and write

speed possible.

Why "random 4K"? Because what we do on our devices is inherently random. Opening an app, updating an app, skipping music files and such. Only things that we do on our devices which are sequential in nature would be recording a video, taking a photo, downloading a large file and so on. (or transferring big files PC<>phone)

Sequential read/write speeds with SD cards (or SSD or HDD) are what's advertised most of the time and those speeds are already waaay more than enough so no need to focus on those aspects!

For example phones record 4K video usually at about 40-65

Mbit/s

which is 5-8.1

MB/s

. And that's sequential, not random.

That's about the heaviest sequential write operation we do on our phones. Downloading a large OS update could be "heavier" if you have really fast internet speed.​

For random 4K read, I consider 10MB/s or above as really good. 8-9MB/s or below, I personally wouldn't bother.

For random 4K write, at least 1MB/s. 2MB/s is nice and above is excellent. (biggest impact=App install/update, OS update etc)

For speed reference and a reason for the above speed recommendations:

Anandtech's internal NAND random 4K speed tests from Galaxy S7 review part 1.

​

You can for example google "make model crystaldiskmark" and see the results.

If you don't find CrystalDiskMark results for some specific microSD card

a) don't bother because it's some random card

b) rummage through Newegg or Amazon etc buyer reviews. Occasionally some kind soul will benchmark his/her recent purchase and include the numbers in his/her review.

For example the Sandisk Extreme microSDXC 64GB is really nice

Kingston UHS-I U3 microSDXC 64GB also looks nice at the first glance but look at the random 4K write speed, that's horrendous! (don't even touch it with a ten foot pole)

Lexar 1800x microSDXC 64GB is one of the best I've ever seen. But it's so expensive (100€'s) ;(

​

Disclaimer: when you are looking at CrystalDiskMark results for 4K speed,

do not look at the QD32 or Q32Tx numbers.

Only look at the 4K speed numbers that don't have the QD32 or Q32Tx text next to them, that's the number we are interested in.

QD stands for Queue Depth. Long story short: Queue Depth means how many read and/or write (I/O) operations are queue'd one after the other.

QD32 is a REALLY high and ridiculous number. You do not see that kind of queue depth with I/O operations on a phone, not on a PC, and heck, most of the time not even in servers either.

The speed number which does not have QD32 or Q32Tx text next to it means that the Queue Depth is 1. Only 1 read or write operation is queue'd. That's the normal kind of QD you see in phones or PC's.