So my question is since using older SD cards the viewing in camera and deleting has slowed down alot, so my guess is the cards are not quite fast enough for the D7200 but do take and store images. Is the MB/s' number important when buying. What is a good SD Card to get, I have always used SanDisk but have seen another brand out their called Lexar.
Paul, SanDisk is not a bad choice, especially since Lexar's past owner, Micron, just sold the brand to a largely unknown outfit called LongSys. Let's get something clear, though: the write speed of the card determines the continuous shooting speed of your camera once it filled up its internal buffer, so a faster card means more pictures per second or minute. However, the speed of the card has nothing to do with the issue you ran into. A card failure is a fairly rare thing but happens at times. It is no more likely to occur on a D7200 than on any slower camera, though - any camera will wait for the image writing to complete, and the card directory to be updated accordingly, before starting to write the next one. (Again, that is after the camera first filled up its internal memory buffer, which is much faster than any external card could be and thus allows for very high shooting speeds until that buffer is full.)
Here is
some fairly old information about card write speeds with the D7200. What it tells you is that if you care about write speed = continuous shooting speed, getting a card spec'd at 95 MB/s will be advantageous. These cards usually also show a "UHS-I" marking, the faster standard your D7200 supports, and have become fairly cheap anyway.
Happy shooting!