I'm not sure what you don't get here, but putting a slow, and/or low capacity card in, with a faster higher capacity and attempt to write images to both, you will store images at the same rate as the slower card.
Oh, I get that, and that's not at all surprising to me. But write speed isn't at all what I was commenting on.
I read the OP as asking, for example, what happens if he puts an 8GB card and a 16GB card in a GH5, with the dual card function set to "Backup Rec," which records the same images to both cards (so there is a backup of every image in case of card failure), and then he shoots 8Gb of images and the smaller card fills up. I don't know the answer, but the possibility that makes the most sense to me is that the camera then ignores the full 8GB card and starts to write any new images only to the 16GB card that still has some space on it (hopefully giving you some warning that "card A" is full).
You say that the camera will continue to write the new images to BOTH cards, and that on the full 8GB card some of the images already stored will be overwritten by the new images. At least I think that is what you say would happen. And I consider that to be unacceptable. I would much rather know that what is already recorded to the card will not be destroyed by overwriting, and any new images will only be written to the larger card.
And, yes, it would of course be best to use two identical cards to avoid the issue.
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Brent
The camera does not know what you want. Maybe the scene just got more exciting, and you'd rather record that, in that case you'd appreciate that the camera will overwrite the previous images. Cameras are computers, not people. They can not think for you.
Try this, take a card that is almost full, and put it in your camera. DO NOT FORMAT IT, just continue shooting until you know that you've surpassed the cards capacity. Then read the card on your computer. See which images are saved, the new ones, or the old ones. After you've done that, please get back to us with your results.
Just an aside here, but since Digital became reality, the first thing that we noticed, is we no longer have a limit of 36 exposures on each roll of film. (many times we rewound, and reloaded with 7 or 8 pictures left, because we were coming to the Bottom of the 9th inning, and we didn't want to get caught reloading, while the action was happening). But the cost of $6 $8 or $10 per roll were over. We could download, Format, and use each card over and over again. (I know people that only own 4 or 5 Memory cards, talk about being cheap!) But since Media became so cheap, most of us own Dozens, or even Hundreds of Memory Cards
I have a Box of 4GB CF Cards, and a couple of boxes of SD Cards, also 4GB. This happened, because the memory went on sale, and we weren't sure if they got cheap, or these were the new prices. I have many memory cards, with images that were shot over a year ago, and the tab is locked. So I have become use to using the cards themselves as a back-up.
I use to have many cards with 16MB, then 32mb, even 64mb. I have boxes of them. My camera is 36MP, so I now use 128GB, and 256GB Cards, whether SDHC, or Compact Flash cards, they have become extremely cheap. Photography became cost effective for once. The bad side of this, is Wedding Photographers that once shot 80 or 100 images for the event, now shoot 2, 3 even 4 thousand images per wedding! I could not imagine sorting through these images. Of course the better Wedding Photographers, only shoot 800 or 1000. I think even that is overkill, but nobody goes to Photography School anymore. Why should they, when their Sony locks onto the Model's eye, and stays focussed on it, even if the building goes on fire! They've effectively killed off the Trained Photographer. Who needs skill, when Sony has it right out of the box. The second I see Sony, I know exactly what kind of event it is. I am sure, that while some of my Memory cards are antiques, but they will actually work, if you wanted to store 1 or 2 images. I shoot Nikon, and I'm very happy with the D810, but the D850 to me, is way over the top. I think they've taken it too far. I'm sure they've got the same thing over at the Canon camp.
****NOTE: MB AND GB Here****
My idea, is somewhere in-between. While I put some large Cards in my cameras, I've never taken images into the thousands. Maybe into the 100's, like 110 or 120, but there is always plenty of media to hold many more times the images than I will take. For an enthusiast, a 32 or 64GB card will suffice. I would say to have 2 of each, to match, or get them at least close, to a camera with two slots. B&H Photo pairs them up pretty well for us, like a pair of 64GB Cards, one for Compact Flash, and one SDHC Card, that match in size, and are very close in speed. This, they sell for $99 and quite honestly, it's all that you need. If you had a second set, for back-up, it is understandable, but these days Back-ups whether Gear, Media or Flash, rarely if it ever gets used. I have a Back-up D700 that hasn't been used in years! In it's time, it was Great, and it still does a helluva job, but why use that when I have the D810?
I do have GAS (Gear acquisition Syndrome) and my latest toy, is the Panasonic Mirrorless cameras, They are quite amazing, and while it is cheaper and lighter to get into, the sensors are smaller. It doesn't matter to the people I shoot for, and the enlargements have great resolution, and color. The HDR, is like something in my dreams, just a few years back. I do recommend m43, for anyone who's looking to get into Photography, but doesn't have a fortune to spend on it. It is extremely cost effective, and anyone that sees the images from the 100-300mm or the 100-400mm Dual IS system, will be amazed. It has 5-Axis IBIS, and add on the Lens Stabilization, and you have both systems working for you, in tandem. This, is the way that they planned it. It took a few years to come around, but it is really fantastic! I really couldn't ask for more. A few of the less expensive ones, like the DMC-G85 have the 16MP Sensor, but that never bothered me at all. The GH5 Brings it up to 20MP's, but I think that was a bit of, "Keepin up with the Joneses", Don't be surprised if you see me shooting, and ask to see my memory card, and you see I've got a pair of 64GB Cards, because that is surely enough for me. As much as I love Photography, I'm not going through 2000 images, to fill a 100 Album Book. Hire one of those women with an accent, because I won't do it!
Back to that original question, check the size of the files you are capturing. If you want to use them for the internet, 8 or 10MP's are considered too large for many websites, and way to large for all email servers. When you turn it down to size, you'll see that they write much faster, and there will be like 3 or 4 times the amount of frames that you have left. Start there, because that's the main place that you're burning your media cards. After that, wait for a sale, like Black Friday, or Cyber Monday, and then get a decent set, of Good Brand cards. If you're storing them on Flickr, they're cutting them down anyhow. All of you, should realize that you do not need such resolution, unless you're printing for a Billboard. You're computer will also have some breathing room too, so cut those down to size. Unless you're shooting the World Series, chances are you're using files that are many times, of what you need.
When I shoot many, or High Resolution images, it's because it's a Boxing Match, a Major Sporting event, the most beautiful woman I've ever seen, or it's because they are actually using the image for a huge (maybe not a billboard) but they're paying for the best that I can deliver. Once you get your Memory card down to earth, check your computer, and see how much space those images are taking up. Of course these companies want us to upgrade every year or two, but if you follow their plan, your retirement account won't have any money in it. Try to be thrifty, even if you have to start with a used camera. Mob.com, B&H, Sammy's, Robert's and KEH all put at least a 6 month warranty, and if you do go that m4/3 route, you will save a ton of money. That smaller sensor, requires smaller lenses, and all of this save us money. Probably half the amount of Nikon, Canon and Sony. (Sony seems to be a subscription service, as they want you to buy new, about every few months. You've seen the stories here, get an old rig, and see how far you can take it. Surprise us, even more important, surprise yourself. Believe me, you will find corners to cut, all over the place.
Just remember, for most, it is just a hobby. If you are Chase Jarvis, then you are within the 5% of us, that really makes it in Photography. Oh, don't get me wrong, you can do Real Estate, or work for a home town newspaper, or like me, you could try Forensic Photography but that will surely take ALL THE FUN OUT OF IT, (So don't go in that direction). ... JP
