The SD card slot in your phone is intended to be used only by the Original Equipment Manufacturer (OEM) that built your phone and your Mobile Operator (MO). These partners can add an SD card to this slot to expand the amount of storage on your phone.
Determining whether an SD card is Windows Phone 7 compliant is not a simple matter of judging its speed class. Several other factors, such as the number of random read/write operations per second, play a role in determining how well an SD card performs with Windows Phone 7 devices.
Rememer our story with HTC and their hidden-to-death micro SD card slot? Looks like they have all the reasons to do so. Here is the Samsung Focus and AT&T’s been telling their employees not to install anything to the slot as it would cause reliability issues. In addition, expansion is apparently unsupported by Windows Phone 7, thus the hidden slots employed by HTC. The main issue lies within the performance of the micro SD cards they say. Only the fastest cards would work flawlessly and more often that not, current classes of cards can’t meet these demands. So why rely on those SD cards if they can just put in an internal memory or something?
So based on this, they say that ratings for the different SD card classes aren’t enough basis in determining if you can use a particular card or not. Looks like Microsoft has to sort this out as SD cards have been used long before they planned on releasing their own phone.