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I am experiencing really slow write speeds on my MacBook Pro on a MicroSD from Samsung which should allow write speeds of up to 20MB/s. The current transfer rate seems to be around 3-4MB/s on HFS Journaled, and the rate was way lower when I formatted it with exFAT. I see some methods for increasing speeds on other devices like Android mobiles, which involve changing the cache, and also on OSX but only for command line processes? Is there a way to implement this when dragging files over with Finder?

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  • Turns out I had bought a counterfeit chinese card that had poor performance :) Mar 16, 2017 at 12:41

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Indicated write/read speeds are always higher than what you should expect from a card.

Usually, the filesystem doesn't make a difference to the RW speeds.

Do also remember that copying many small files is significantly slower than copying one big file due to the entries required to the FS.

If you'd like to test your write speeds with the filesystem, try

dd if=/dev/zero | pv -betrP -s 100M -S | dd of=/path/to/your/device/temporaryfile 

The above command will dd 100 mebibytes of zeros into a file into your microSD card, and will return a speed in MiB/s.

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If you have many small files to transfer over, avoid the processing overhead the Finder imposes on file drags by using the Terminal command cp. Type man cp for all the details. Should you be using the MicroSD card as off-line archival storage for many small files, consider compressing the set of files into a zip archive, then transfer the .zip file to the card, which will copy over much faster.

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