I have 2 external Seagate 3TB, USB 3 drives connected to my mid 2011 mac mini. I assumed it would be faster to have each one connected to a different host controller (usb 2 on my mini). My assumption was correct.

Sharing the SAME controller:
- Code: Select all
dd count=1k bs=1m if=/dev/zero of=test.img
14.86 MB/s
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dd count=1k bs=4m if=/dev/zero of=test.img
14.37 MB/s
Connected to DIFFERENT controller:
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dd count=1k bs=1m if=/dev/zero of=test.img
20.57 MB/s
- Code: Select all
dd count=1k bs=4m if=/dev/zero of=test.img
18.75 MB/s
In the end the only thing that surprises me is that there isn't a bigger difference between the results. Although, 25% increase is respectable.