What happens when you place the suspect SD card into the USB card reader or better yet a SD card slot of your PC? Does the PC see the card? Are you able to view any of file contents on the card? Do you get a message that the contents are corrupt, unreadable , or needs to be formatted before it can be used?
Have you tried powering on the unit with a different , blank formatted SD card in the slot to see if the DS 708 can at least be able to read and determine that some sort of blank SD is inside the slot?