Is there have way to know a StorageFolder-obj belongs to SD or Phone on WP8.1? Thanks you.
You can do it for example by checking the Path of StorageFolder (or StorageFIle):
string folderPath = yourStorageFolder.Path;
if (folderPath.StartsWith("C:")) // on Phone
else // on SD Card
D: (or other letter - not C:) - it means it belongs to SD card,C: - it exists on Phone,EDIT - as Adam has said in comments - it may be more suitable to check first if file is on the Phone, in case the System had assigned other letter than D: for SD card.