I'm just about to start writing the OctoPi image to the supplied SD card. Loaded it into my USB attached card reader on Windows and there are files on the SD card already. Is this the image I need without overwriting it? If not what is supplied and why? There are instructions on the SD card which suggest using another? SD card to write an image to... whereas if you followed the BB build instructions you would overwrite this card.... and the instructions on it. Plus Windows says that this SD card is less than 1GByte and the image is I think at least 3 GBytes?