Hello

After reading most of the posts it my thoughts the card might still be good. You did tell us what type card you're using is it a CF or SD card if it's a CF maybe bent one of its pins when you put back in your camera if not the case off load all your photos and reformat it. If it's a SD card off load your photos and clean the contacts on the card and reformat it too. If these things don't work as said before contact Sandisk for a replacement . Until then us one of your other new cards and do a format the card in the camera.

Computer guy here. All flash memory, whether camera cards or thumb drives, will degrade over time as each sector is written and rewritten. Software in modern cameras and computers helps to even out this wear, but eventually, they do start to degrade. They're so cheap now, it doesn't make sense not to replace them at the first sign of problems if reformatting them doesn't straighten them out. Your second concern about long term storage of new memory cards is nothing to worry about as long as the contacts haven't oxidized, in which case a quick, careful going over with a pencil eraser should clean them up. Storing them doesn't "drain" them, the amount they have been used is a factor though. Hope this helps.

For most people, I think the D610 is the best FX choice.

The D800 has 1.5x more mp, but I think that is a small difference, definitely less important than sensor size. More mp means higher resolution, but only by 22%, but since the pixels are 1.5x smaller, there is some image quality loss from that. The more important advantage for some is that the D800 has a better AF system, with the same AF sensor as the D4. Of course, the D800 costs 1.5x more than the D610, and definitely is not 1.5x better.

The D4/D4s are in the $6000-6500 range, and if you were seriously considering those, you wouldn't also be asking about a new DX camera. ;-)

I think the Df is a good camera for one person: the guy who has a bunch of old Nikon lenses from the 60s and 70s, because it works with non-AI-s lenses. Otherwise, it costs as much as the D800 but is closer to a D610. Yes, it has the same sensor as the D4, but so what, that isn't the best Nikon sensor, the D800 is.

FX is better, but only incrementally. Are you
1. Shooting at ISO 800 - 6400 a lot?
2. Enlarging to large sizes like 20x30 a lot?

The simple comparison is between the D7100 and the D610, both 24mp. The D610 sensor is 1.5x larger linearly and 2.25x larger by area. The D610 pixels are 2.25x larger, which improves the high ISO noise as well as dynamic range and color depth. Since the FX sensor is 1.5x larger, the DX image needs to be magnified 1.5x more to make the same print, which results in some loss of print quality.

FX is better, but only incrementally. Are you
1. Shooting at ISO 800 - 6400 a lot?
2. Enlarging to large sizes like 20x30 a lot?

The simple comparison is between the D7100 and the D610, both 24mp. The D610 sensor is 1.5x larger linearly and 2.25x larger by area. The D610 pixels are 2.25x larger, which improves the high ISO noise as well as dynamic range and color depth. Since the FX sensor is 1.5x larger, the DX image needs to be magnified 1.5x more to make the same print, which results in some loss of print quality.