Not sure this is really a D3100 issue so much as a memory card issue (likely corrupt), but posting here for any wisdom you all might be able to share. I was shooting today at a family outing when all of a sudden I got this error "This Memory Card Cannot Be Used. It may be damaged. Use another card". That may not be the exact text, but it was something VERY similar and I'm sure the first sentence is correct.
Anyway - another, identical SD card works just fine, but this one cannot be read any longer by the camera or any of my various SD cards readers (one built into my printer, one built into my laptop..both which can read other SD cards just find, including others that are identical in maker and specs to this one).
Tried to recover the data on it, but none of the software I downloaded can even get tot he point of recovery because none of the readers are even able to recognize the card.