It's best to not delete in-camera. Here's how it was explained to me:
Think of your memory card like a piece of notebook paper. When you're taking notes, you can write on the lines and everything is easily readable from beginning to end.
Now you reach the end and there's not more room to write. You may write up the side, in the margins, or between lines. Perhaps you draw lines and arrows to connect things you've written down so that they make sense later. You may even erase something you've written in order to write something else in that spot. Erasing can rip the paper or cause confusion as you try to read what you've written over something else.
With a freshly formatted memory card, all the available space is clean and your camera can write on it easily, just like a clean sheet of paper. If you delete (erase) in-camera, there are now those little open spots where your camera can write in the margin. It draws an arrow to let it know where it left off, but sometimes erasing doesn't allow it to be read easily.
Format a new card before each session. Carry additional blank cards. This will keep you from worrying about deleting info you still need space.
Before a memory card goes into my bag, it needs to be clean and formatted. That way, if I put it in my camera and see data, I know not to use it because it's probably not been downloaded yet. Yeah, I carry a ton of cards . . . but I'd much rather be safe than sorry.
Finally, remember that memory cards do have a life expectancy. It's a good idea to number them and replace them every couple of years. Sure, it's an added expense, but we're in business. It's like having backup gear in your bag or backing up files to external drives or cloud servers. Figure it into your cost of doing business.
- David
