When a card is formatted and you start shooting then you fill up the card from start to finish consecutively. If you delete the last image then it just continues on. If however you delete an image that wasn't the last one then there is a hole that has to be filled.
Hi Bob,
That's not exactly how things go on a memorycard / memory stick / SSD. Since these devices only allow a given number of rewrites to each memory position, maybe 1000 - 5000, they will spread those write operations uniformly over all available positions over time. This is called Wear-leveling. So a deleted file position will be marked as such, but will probably not be re-used again for a while (if there are other positions available that have not been written to for a longer period of time).
There will be a certain fragmentation, but in solid state memory that hardly affects speed. Large memory devices like SSDs can be sped up a bit after a while by using a so-called TRIM operation if the Card OS supports that, because defragmentation would not really work, and it would needlessly use up the maximum number of writes that are available.
Memory Cards can be (low level) formatted, which will reset everything (possibly also the Wearleveling). So it is probably best not to do that too often. Simply deleting the files (with a reader connected to the computer if bulk deletes are done) should do the trick.
Cheers,
Bart