597 images on a 32 bit card is correct if you are shooting RAW only and saving the images as lossless compressed 14 bit images. See the tables on page 348 of your manual (the numbers are based on 8GB cards so adjust accordingly). As stated by others ISO and read/write speed of card do not impact the number of images the card can hold. ISO setting shouldn't impact formatting either. If you only have one card in the camera are you somehow telling the camera to format the card in the other slot (which doesn't exist). Reading the manual it doesn't look like you can do that but you may want to check. If you are only using one card make sure it is in slot 1. Again, I don't know if this is your problem since I always have 2 cards in my D7100.