There has been a lot of speculation and misinformation about how Windows Phone 7 (WP7) handsets use SD card memory to expand system memory. According to a recent Microsoft support document WP7 treats SD card memory as a permanent integrated component of your phone and not as removable expandable storage.
The support document also states that when you start your phone for the first time, or when a hard reset (factory reset) is performed, the WP7 OS reformats the SD card to a proprietary WP7 format. It then locks the card to the phone with an automatically generated key and creates a single file system that spans the storage of the card.
After the SD card is integrated with your WP7, the phone’s operating system uses all available memory on the card as a single storage space. Once the card is locked to the WP7 device, it is secured and you will no longer be able to read the contents of the card on any other device.
It’s clear that you will lose any data stored previous to the factory resetting your phone; what is not clear is whether users will be eligible to reinstall previously purchased titles to their newly reset phone. If reinstallation is permitted, it’s still not clear how this process will be handled.
It also was incorrectly reported that once a card was locked to a WP7 handset it was permanently tied to the device and could not be reformatted and reused. Several sources are now reporting that the card can be reformatted using a Nokia N8 handset. I speculate that upon further investigation other methods of reformatting WP7 SD cards will be discovered or developed.
While waiting for Microsoft to certify SD cards to work with Windows Phone 7, Sandisk released a recommended premium priced group of SD cards for the Samsung Focus, ranging in size from 2GB to 32GB capacity. A day later, Sandisk pulled the recommendations from its site without explanation. (see screen captures below)
There also have been reported compatibility and performance issues regarding SD card installations. It has been a “hit or miss” scenario in which users have reported mixed results using the same speed class cards from the same manufacturers. At this point, it may be best to wait until there is a response from Microsoft regarding officially certified WP7 SD cards.