People usually view images in the Playback mode. The photos from both SD cards show up the LCD screen. By alternately pressing the + or - buttons you enlarge a single image or shrink it. Shrink it a few times and you see a calendar with days of the month. All the images taken on a given day are contained in one of those days.
Select a day and then hit the TRASH button next to > (playback) and you delete all images taken that day, regardless of which card they were on.
If you scroll singly through the little icons taken on a given day, you can trash them selectively. The camera navigates through card 1 first. When that is empty, the next images are on card 2.
So, you are saved from making mistakes in discarding important files. Trash a whole day when you are sure of what you're doing and in a hurry. Trash the images singly when you have leisure time. Both methods are needed at different times.
You can switch quickly between card 1 and card 2 for playback on the camera if you need to. In playback mode, if you press the BKT button and the up arrow on the multi selector, you get the option of changing playback slot and folder. It's also a very quick way of navigating around if you use multiple folders.
Deletion is card specific. If you're reviewing card 1 and you delete an image, it only deletes that specific image on card 1. If you want to delete the corresponding image on card 2, you need to review card 2 and delete the image from there.
I have read in several places that it is better not to delete photos individually the camera, but rather format the card AFTER you have downloaded the photos. Something about corrupting the card?? Maybe someone that better understands these things can jump in.
Jerry Jaynes Great Smoky Mountains of North Carolina
>I have read in several places that it is better not to delete >photos individually the camera, but rather format the card >AFTER you have downloaded the photos. Something about >corrupting the card?? Maybe someone that better understands >these things can jump in.
That's basically nonsense. The "file system" used on the cards is similar to one used on disk drives. In fact, some early CF cards were tiny disk drives. There was some value back then to not fragmenting the disk storage by deleting individual files. It had nothing to do with corruption but with performance. But those issues don't apply to solid-state cards.
The only reason I know of not to delete images from a card is that hastily doing so in the field may lead to deleting images you didn't mean to delete. That may be a good enough reason by itself, but on a few occasions when I unexpectedly found myself running out of storage, I've deleted images with no ill effect.
First, read cycles have no effect on the life of the card.
Second, while it is true that cards have a limited number of erase/write cycles per block, the number for modern NAND flash is in the range of 1,000,000. Wear leveling ensures that writes are distributed throughout the card. You would have to use a single card through the life of about 4 pro DSLRs' shutter lifetimes before approaching the limit, if then. So that's not a realistic consideration. (The manufacturers actually specify lifetime as mean time between failures, MTBF. I believe SanDisk specifies 1+ million hours MTBF for their consumer-grade products.)
Finally, fragmentation can affect performance due to the block nature of writes, but it is significant only when there are many small files. Because the files we store on a card in a modern DSLR exceed the size of the write blocks by many times, the files contain only a neglibible percentage of partially filled blocks.
As for fragmentation "increasing the risk of errors," please explain the mechanism by which that increased risk occurs, remembering that these are solid state devices.