Explained: Windows Phone 7 microSD card: why you can't change it

Microsoft has been saying all along that even if you can get at the microSD slot in a Windows Phone that the point isn't to be able to swap cards.

As general manager Charlie Kindel told TechRadar: "It is possible for the manufacturer to put a microSDcard in the phone - but that memory is not user serviceable, you can't pull it out or replace it."

The quick start guide tells you that if you do take the card out, the phone will reset itself and you won't be able to use the card in other devices.

That's because the OS pairs the card to the phone by locking it with an automatically generated password and checksum - which means it can recognise the card and be sure the content hasn't changed.

Why does Windows Phone 7 do that?

"We use the built-in flash memory on the phone and the flash on the microSD as a single file system," Kindel told us: "if the user were to pull it out, it would break that file system."

Your music, pictures, apps and everything else will be spread across both areas of storage and the phone has no way of knowing what's on the microSD card so it can't just tell you something is missing - it has to reset the phone to make sure you don't try to use something that's no longer available.

Setting the password isn't permanent - but the vast majority of devices don't implement passwords for SD cards so they won't even recognise the card to format it (some Nokia phones support this part of the SD card standard).

Windows Phone 7 microSD expansion

Suppose you want to replace a low capacity microSD with a larger one and you don't mind resetting your phone? After all you can get your apps back from the marketplace and your media from the Zune software.

The problem is, according to Charlie Kindel, "these memory cards have very different speed characteristics and if the user is willy-nilly putting in different memory cards that would hurt the phone experience."

Having a slow SD card in your camera will mean it takes slightly longer to save a photo but you might never notice; having apps launching from a slow SD card could make your Windows Phone feel very sluggish.

Why can't you just buy a fast microSD card of the right class?

"Yes, it's about read write speeds [which is what the SD class number indicates]," senior product manager Greg Sullivan told us. "The IO rate is part of it but actually what's equally important are the bit error rates. That will impact the speed of the checksum rewrite."

Microsoft was hoping that the issue wouldn't come up for most users (and we agree that the majority of mainstream users will never open up their phone to change the memory card).

"Most of the microSD slots are not user accessible for that very reason," Sullivan told us. When they are, as with the Samsung Focus, "there's a big sticker that asks you to read the Quick Start guide before changing the microSD".

Larger Windows Phone 7 microSD cards

The obvious answer is for Microsoft or the handset manufacturers to test and certify memory cards.

But the Windows Phone team told us they didn't have the time or resources to do this before launch, that manufacturers tested only the microSD cards they buy in bulk, which aren't available in retail - and that just going by what the chassis specification for the handset says (or even the class of the microSD card) isn't enough to guarantee good performance and performance varies between vendors and even between different cards from the same manufacturer.

The same member of the Windows phone team told us they're now busily buying every microSD card they can get their hands on and testing it in their phones, and when they have a list of which cards perform well enough to recommend they'll be distributing the information.

But for now, the official Microsoft line is still not to swap out your microSD card for a new one.