Huge problem with image area in d800

I bought a DX lens and the person who sold it to me, set my D800 on DX crop mode. I’ve then taken many pictures on this mode, little realising that I was losing most of the picture! A proper image was visible in the optical viewfinder but the pictures were hugely truncated, making them worthless.

CRUCIAL TO CHECK THE FOLLOWING MENU OPTION EVERY TIME BEFORE SHOOTING:

1. GO TO IMAGE AREA
2. MAKE SURE THAT AUTO-DX CROP IS OFF
3. CLICK ON CHOOSE IMAGE AREA
4. PICK FX ALWAYS