Microsoft has posted a Knowledge Base article, Windows Phone 7 Secure Digital Card Limitations, which seeks to explain its stance on this thorny issue. As with all things Microsoft these days, it does sheds some light but leaves the central question unanswered.
As usual, the bolded parts are mine, for emphasis.
Some Windows Phone 7 devices include a Secure Digital (SD) card slot underneath the battery cover. If you buy a Windows Phone 7 device that includes an SD card slot, you should be aware of several important differences from other devices that use SD cards:
- 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.
- To help ensure a great user experience, Microsoft has performed exhaustive testing to determine which SD cards perform well with Windows Phone 7 devices. Microsoft has worked closely with OEMs and MOs to ensure that they only add these cards to Windows Phone 7 devices.
That bit, of course, is new: We knew Microsoft tested cards. But now they are apparently OKing certain cards. I bet you can’t wait to see which ones they recommend…
Performance Requirements for SD Cards
The SD card in a Windows Phone 7 device must meet certain performance requirements for the phone to function optimally. If the SD card does not meet these performance requirements, the phone will have portions of memory that meet the performance needs of the operating system (the internal storage) and portions of memory that do not meet the needs of the operating system (the SD card). This can lead to poor and unpredictable performance when using the phone. This article uses the term Windows Phone 7 compliant to refer to SD cards that meet the performance requirements for Windows Phone 7 devices.
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.
And there you have it. Microsoft has identified which cards work. Microsoft has not provided that information to the public.
Round and round we go.
Thanks very much to Mike S. for the link.