You need to use Win32 disk imager if you wish to make a backup of your SD card.
Step one: Place the working SD card in your pc and open disk imager select the image file you wish to make.
Step two: make sure the drive letter is pointing to the SD card not another drive.
Step three: click read this reads the SD card to the file you specified
Step four: when done pull the SD card and place the one you wish to write to in the PC.
Step five:make sure the drive letter is pointing to the SD card not another drive or you'll write the image to that drive (I've done that)
Step Six: Be sure you have the correct image to write selected (if you didn't close dis imager it should still be displaying) and press write