I checked Windows Phone 8 SDK document, and find that we can read the data from SD card, but we can\'t write something to it, is my understanding right?
You can get read-only access to specified files on SD cards using the ExternalStorage
API and ID_CAP_REMOVABLE_STORAGE capability, but you cannot write to this storage.
http://msdn.microsoft.com/en-us/library/windowsphone/develop/jj720573(v=vs.105).aspx