SD Cards in Windows Phone 7

There have been a lot of questions on forums about the SD card support in Windows Phone 7. The SD card is not used the same way in a Windows Phone 7 device as it is in many other smartphones, and this can cause some confusion. In this post, I don’t want to discuss whether this change is a good idea or a bad one. I just want to dispel some confusion by stating the facts as I understand them. I’m not an official spokesperson for the Windows Phone team, but I have some information that might be helpful to the average consumer.

The first point is that the SD card should be thought of as if it were your phone’s hard drive, not as removable storage. Not all Windows Phone 7 devices support replacing the SD card, and if the device does support replacing the SD card, you’ll have to perform a factory reset (losing all of your settings and saved files) as part of the replacement process. The phone won’t work correctly if you remove the SD card. Replacing the SD card on a Windows Phone 7 device is like replacing the hard drive on a desktop PC.

It follows that a slow SD card will have a big impact on the overall performance of Windows Phone 7. Adding a slow SD card to the device will slow down the whole phone. Unfortunately, determining whether an SD card is “fast” or “slow” requires a lot of testing, and the advertised “class” of an SD card has almost nothing to do with how well it will perform in a phone. SD card “class” measures how quickly an SD card can save a single large file such as a photo (think of this like you would think about a car’s top speed). Windows Phone’s performance is more affected by how quickly the SD card can load and save a whole bunch of tiny files (think of this like you would think about a car’s performance in an obstacle course). Don’t fool yourself into thinking that any old SD card will work or that a higher "class" of SD card will make your phone work better. Microsoft tested a whole bunch of different SD cards. Very few performed well enough to be approved for use in the phone. SD cards that are not "approved" may work correctly, and may even work well, but they may also work poorly or not at all. Unfortunately, the SD card’s “class” won’t be very helpful in predicting how the card will perform in the phone.

Finally, the SD card is locked to your phone, and data on the SD card is encrypted. The data on the SD card can only be read by the phone, and cannot be read by other computers or phones. The encryption key is set when the phone first boots after a factory reset. Once the SD card has been locked, there are two ways to unlock it. The first is to provide the encryption key, which only your phone can do. The second is to perform a low-level reset on SD card, which erases all of the SD card's data. The phone will perform a low-level reset on the SD card during its first boot after a factory reset, and will then lock the card to itself. Unfortunately, I don’t know how to do a low-level reset on an SD from a desktop computer. This means that once your SD card has been locked to a Windows Phone, it won't work anywhere else. (In the future, tools might become available to reset the SD card, but for now, you should assume that the SD card is no longer usable.)

Official Microsoft KB article:

Hope this helps!

2010/11/10: Minor editing, added link to KB article.

Comments (3)
  1. Joe Z. says:

    Thanks but this article repeats information that I think is already clear from many, many sources. We get that the SD card integrates with existing memory and can't be swapped at will. We get that cards meeting strict characteristics are needed. Here is the problem:

    A major Windows Phone 7 device, the SAMSUNG FOCUS, was sold by Samsung and AT&T EXPLICITLY AS OFFERING UPGRADEABLE MEMORY. (In fact, that's the only justification for shipping this phone with only 8gb internal storage, when every other Windows Phone 7 has 16gb.) It doesn't matter what Microsoft now says, because the official documentation with the phone tells the user exactly how to insert a card, reset the phone and format the card. What the documentation DOESN'T SAY is that the user has no way to knowing which microSDHC cards will work, and which won't. Now AT&T is backtracking on the upgradeable-memory statement, but it is too late for that– the phone was already sold and packaged as having that feature.

    SO: Microsoft, Samsung and AT&T need to get together and rectify this situation immediately. There are exactly two options. Either issue a patch and make microSD cards available that work in the phone (either by sales or through guidance), or offer to accept returns of the phone WITHOUT A RESTOCKING FEE and then change the phone's marketing.

  2. M GILL says:

    Windows have taken one step forward, and ten steps back. First of all, no flash, no copy and paste and

    now we are stuck with ths sd card nonsense. Why are the people at windows 7 team not consumer focused. I have always used windows phone and removing and coping files on to sd card is standard stuff and taking out sd card and using them elsewhere is pretty basic. This was one of the main advantages over the iphone that windows mobile had removable sd card.

    Microsoft are forcing conumers to buy andoid which we really dont want to. Like most comsumers we are stuck in a dilemma. We can only wait and see if microsoft change their mind, if not, then android all the way.

  3. Eric says:

    Any tools in the meantime to reformat a WP7 encrypted SD card on a desktop PC? I don't need to recover anything from the card, but it would be nice to use it again. 🙂

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