dd just blindly copies what you feed it, including the header sectors that include information about the partition n stuff. However, unless the image came from the whole card, it does *not* contain the MBR with its partition table which tells the device where the partition starts. Dumping an image of a partition onto a RAW device will pretty much end up in useless garbarage, one definitely needs to create a partition to overwrite just so the device knows where to search for it.