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I have a 7D MkII. I store on both SD and compact flash, which seem like a good redundancy feature. Thus far I have only been popping out the SD card and plugging it into my MACBOOK to Transfer files. Basically only using the CF card as an emergency back-up.

Does this make sense, or should I get getting a card reader for the CF Card and transferring from that? Would there be any upgrade in speed transfer or image quality?

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    What class SD card are you using? What rating CF card? What is the speed of your SD card reader and connection to your computer? What is the speed of your CF card reader and connection to your computer? Depending on these variables, either one might be faster than the other. – Michael C Mar 25 '18 at 0:39
  • And, with regard to image quality, what settings are you using to "store on both"? If you have it configured to store RAW on one card and JPG on the other, then image quality is obviously a variable, but if it's saving the same formats to both cards, that won't be an issue - they should be identical. – twalberg Mar 25 '18 at 11:56
  • All good questions. Seems like my CF Card is faster and higher quality. I think it's going to be worth getting a 3.0 USB Flash card reader. I'm recording in RAW on both cards to make it a true back-up. – Clay Derouin Mar 27 '18 at 6:43
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Speed depends on the cards and the reader (or writer in the camera's case). The camera's ability to write to them any faster won't have any effect on the quality of the images, just how fast you can make and store them.

From a reliability standpoint, I'd make some changes to what you're doing.

SD and CF cards use flash memory to store what you write to them. Flash doesn't last forever, and eventually it will start having unrecoverable problems. Exactly when that happens depends on a lot of variables, the biggest of which is how much data has been written to it. For the sake of simplicity, let's say your cards last a year of shooting before they start experiencing errors.

What you don't want is to pull your SD card out of the camera after a year of shooting, find that it's failed and then discover the same thing when you pull out the CF card you're using for backup. That would be bad.

You can avoid getting into this situation by making sure both cards don't wear out at the same time. The easy way to do this is to stagger replacement of your cards by half their lifetime or, if you don't know that, some interval large enough that you won't lose pictures. Installing a new backup card six months after your primary will, all other things being equal, offset its failure date by that much. When your primary experiences a wear-related failure, the backup will have six months' fewer writes on it and will be unlikely to fail for the same reason.

If you do your purchasing on a staggered schedule as well, you're less likely to get two cards from the same batch, which protects you against bad batches and won't lose you any data if you replace one of the cards with one that's bad out of the box or turns out to be a counterfeit that doesn't last nearly as long as the genuine article.

  • Thanks for the comments. I like your suggestion on staggering the cards...makes for a little less chance of dual-failure. – Clay Derouin Mar 27 '18 at 6:44
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It all depends on the models of the cards and more specifically their read speeds. If the CF has higher read speeds, the only benefit you get is that you'll be offloading files from it faster (but this also depends on the card reader's speed and connection). USB 3.0 has the bandwidth to accommodate even the fastest cards available today; 2.0 can potentially be a bottleneck. As already mentioned, if you've set the camera to write the same format to both cards, it won't matter which card you copy your images from - the files will be identical.

Edit/update: Any transfer between devices will be limited by the slowest link in the chain, so in your case, getting a faster SD card reader for example won't help with this particular SD card since that's how fast the card goes. USB 2.0 will be a limiting factor indeed for a CF reader - it's specified at 480 Mbit/s which is practically 60 MB/s or less. A good bet would be to get an external Thunderbolt card reader given you don't use the port for something else and given you can find one for a reasonable price (since they're not as commonplace as USB readers, making them more expensive). It's up to you to decide if you want the speed of an external reader over the convenience of the built-in one. You can still buy a USB 3.0 reader (it will fall back to 2.0 specification on this computer) and although it won't give you any benefit over 2.0 right now, you can carry it over to a newer computer in the future.

  • Looks like the CF is 120 MB/S and the SD is only 30 MB/S. I only have the SD card reader which was on board with my 2011 Macbook and I didnt see any specs on it, but I suspect it's not amazingly fast. And since the Macbook uses USB 2.0 if I upgrade to a newer CF Card reader with a faster speed, it seems like it will be limited by the USB2.0 Connection. I think the answer is a newer Macbook with USB 3.0 and a USB 3.0 CF Card reader. I like your suggestion on staggering the memory cards I use in the Cam; makes sense. Thanks for your input! -CD – Clay Derouin Mar 27 '18 at 6:40
  • I updated my original answer, please take a look at it. – K. Minkov Mar 27 '18 at 7:43

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