I just had a malfunction on a new Kingston 16 GB class 10 SDHC 1 card, in my D7100. After 227 RAW files were captured it read FOR on the top screen, and the camera would do nothing with that card installed until it was formatted. I was able to recover the files with a card recovery program on my laptop, but it shows only a very low compression thumbnail of each image, even though each file title shows average 27MB. I just now found out that Nikon recommends certain brands of SD card, from members in this posting. After formatting the Kingston card, it seems to work fine, but I'm wondering if the same thing will happen as the file count goes up again. Thanks, Steve
The RAW (NEF) files includes both the full raw information, as well as a thumbnail for quickly browsing. So while you are currently only viewing the encapsulated thumbnail, the full information is still there but the file might be slightly corrupted. Some of the RAW editors might just be able to ignore this minor defect (no guarantee, so) but it is worth trying.
First you might want to install the most recent NEF codec
Nikon | Download center | NEF Codec
As regards the SD card it is a consumable - and nowadays quite cheap. While it might work properly after having been reformatted (chances are high), I wouldn’t trust it anymore, and therefore replace it (i.e. I consider my effort to take the captures a higher value than a low cost card.)
As the D7100 has two SD card slots I would use it in the second slot at best and select in the menu that the second card is the backup for the first one. (So, if it fails, it is only the extra backup that fails - and if it doesn’t, this option gives you an extra backup in case your first card fails).
Btw. Regular spontaneous in-camera formatting of memory cards, after having copied all captures, are useful for maintaining a clean filesystem on the the card.