[HTC 7 Pro] eduncan911:
The problem with the Class 6 or higher cards (like your Class 10) is the Random Access Times is too low. It will cause random app crashes (it just closes on WP7, and you click it again to load it again). More horrible cards, the entire Wp7 will just keep rebooting randomly.

You see, with these microSD cards, you have to sacrifice one thing or another: Either the Data Read/Write speeds (the "Class X" number, the higher the number, the faster the reads and writes), or the Random Access times which is the speed it takes to locate and access that data, before it can be "read" or "written" to.

Windows Phone 7 has the lowest access time requirements out of any operating system. Meaning, the storage device (e.g. your Class 10 card) must respond with X milliseconds, or the application will be deemed faulty and taking too long to load.

Microsoft set the bar really really high (very low access time requirements) because they wanted a phone to be ultimately responsive.

Well, they shot themselves in the foot with this requirement because SanDisk was the only one to provide an microSD card with fast enough access times. They are the Class 2 and Class 4s you see in the WP7 phones, and they are becoming hard to find.

I highly suspect that with the first update that came out last year (the "intermediate" one before Mango), Microsoft "loosened" the random access time requirements because memory that fast was becoming very hard to come by. This, in turn, now allowed the apps to respond slower too. This is why you can now install apps that seem to have that slight longer delay - because Microsoft loosened the response time requirements.

With that update, it now allows me to use a Class 10 card that has a very high random Access response time, and this is most likely why your Class 10 card seems to work as well.

Personally, I'd pick a card with a very low random access time, like a SanDisk Class 2 or Class 4 (that I already installed, see sig). I've switched back and forth between my Class 10 WinTech 32 GB (again, see sig) and my Class 2 SandDisk 32 GB and I can tell a serious difference in response times - the Class 2 blows the Class 10 out of the water and is extremely "snappy".

Even my wife can tell a difference with an adhoc blind "how's this for speed?" test I did with her. "What's wrong with your phone? It seems slower now." <- THat was the Class 10 card after the Class 2 card. I went back twice to confirm, and both times she picked the times I had the Class 10 installed.

We're talking on the level of 50 to 150ms. Yes, that may seem very very small - but think if you added an 100ms delay to every "touch" to your phone to scroll, move something, open an item, etc - that's what you experience with a Class 10 card. Putting in a Class 2 card, it's like instant response time.

And you can't tell a difference until you do a comparison of Class 2 to Class 10. Unfortunately, since the phone is locked, I can't do a speed comparison as there isn't anything to measure for how fast an app response to a start. But trust me, there is a big difference between Class 10 (very slow) and Class 2.

I learned this back in my Android days - a high Class X number is only for Cameras that need to write a lot of data quickly. For something like an Android or WP7 device, you want the absolute fastest random access time - and that my friend, are the Class 2 and Class 4 cards.

But good for you for upgrading!

Btw, my HTC Arrive w/32 GB Class 2 SandDisk is for sale.