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I have several Sandisk SD cards all rated Class 10. One card transfer files to my computer faster than the others. Specifically I had taken several pictures and used a different card each day.

One 16G card transferred 528 pictures (10.4G) to my computer in 7 minutes. Other cards took hours. For example, another card transferred 508 pictures (9.8G) in 2-1/2 hours. The other cards I had (both 16G and 32G) also took over 2 hours to transfer about the same amount of data.

What is causing this behaviour?

  • Thanks for introducing me to SD card speed classes (: Apparently the class is only for write speed, though I don't know if that explains what you're seeing. Could you try formatting the cards and retrying the speed test to eliminate fragmentation as a possibility (though I suspect fragmentation would only affect write speeds)? – Andy Jul 16 '14 at 12:46
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If classes are the same, then my first assumption would be fragmentation issue. SD cards are typically formatted with FAT file system which suffers from fragmentation if the card is used intensively over a period of time.

If this is true and fragmentation is the problem, then you need to defragment the card. However do not use any defragmentation software. This software is designed to defragment hard drives and is relying on loop-based algorithms which will wear your card significantly by performing multiple write operations. What I would recommend is to copy all data to backup storage (HDD for example), then format both cards and copy the data back - this should resolve fragmentation issues (if any).

Here is a very good article where author describes his experiments and proves that fragmentation can decrease SD card performance up to 50 times

  • I have very rarely used the card with the faster transfer rate so I can see where fragmentation can be an issue since I use the other cards more often. Thanks for the links to the article. Off to do some reading – Steph Jul 16 '14 at 13:04

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