Memory cards should be formatted on a regular basis. This is to help protect your photos.
Why?
Deleting images is great when you are out shooting and you just want to free up some space on your card, but if you are done shooting for the day, or month, depending on the frequency of your picture taking, it is always a good idea to back up all your images somewhere and format your card. When you delete an image, your camera marks the file as “Free Space” so future images can be saved over top of that. It is not actually deleted. If you only delete your images and never format your card, over time residual image data starts to clutter up your card. This can cause the card to operate much slower and in time it can corrupt your data causing you to lose all of your images. This is why it is very important to format your media card on a regular basis.
ALWAYS format your memory card in your camera, not your computer! When you format your memory card, ALL data gets erased and your camera re-creates the file structure it needs to save your images. If you format your card in a computer, your computer will set up the file structure ideal for a computer, which could render the card useless in your camera.
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