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I went and bought an expensive SanDisk Extreme III SDHC 8GB class 6 chip.

However, when I dump data from the card to the machine via a USB 2.0 cable, it's only getting 5.0 MB/second maximum according to Windows 7's disk explorer. It still can take up to 20 minutes to dump the card when it's near full.

This is so far below the rated 20 MB/s transfer speed. I can't believe it.

Is this normal or might I have a defective chip?

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Slightly off-topic but I think it is still relevant: I have seen different card readers make a 10x difference in speed. If you have any other readers around, that may be worth testing.

Also, the Windows Explorer reported speed works funny - there is a ton of buffering and other I/O magic that happens that really can hide performance.

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Here's a review comparing SD and CF memory cards, which shows SD cards like yours getting 14.5 MB/sec or so. Admittedly, these benchmarks are on a Mac. Perhaps your SD card reader or CPU is the bottleneck?

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This class of SD card should support a minimum transfer speed of about 6MB/s, which would agree with your test of 8GB read in 20mins (6.67MB/s mean). However, I believe the manufacturer is free to quote any maximum transfer speed they can come up with (probably the maximum sequential read speed under ideal, de-fragmented conditions).

I suspect this may be the limit of the card, but have you tried formatting the card with a different file system?

So, based on the link you provided, you could possibly argue with Amazon/the seller that the product description was misleading - it did not specify that 20MB/s was the maximum sequential read/write speed.

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