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Say, I want to copy the contents of a USB 3 hard drive to another USB 3 hard drive. My computer has USB 3 ports, so that's ok.

Would the internal drive be a possible bottleneck? I.e: While transfering files from one usb device to another, does some of the data pass through the internal hard drive, causing a bottleneck to the transfer rates?

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USB 3 is no longer limited to the slowest device. This means a USB 3 device can write and read as quickly as it can and won't effect any other device. Since you are going from USB 3 device to USB 3 device this is less true but bandwidth shouldn't be a problem. –  Ramhound Sep 19 '12 at 11:38

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The internal drive should not get involved.

If you copy from A to B it will not pass through C. It will however pass through RAM-memory, and maybe cpu.

RAM-memory is used as a cache/buffer to speed things up. (internal hard-disk is not)

This answer is for Gnu/Linux, but I think for even the most brain-dead operating system it will not involve the internal hard-disk.

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Great. Thanks for the answer. –  Dashie Sep 19 '12 at 9:48
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Depends on the write cache settings and speeds, it may have to cache some of the write queue due to the buffer filling. It would all depend on the read/write speeds of your devices. Most NAND will read much quicker than it can write. –  HaydnWVN Sep 19 '12 at 10:35
    
Does it, depend (I am assuming you are talking about whether the computers core memory/RAM is used as cache/buffer)? If it is SCSI then the computer can instruct the devices to do a device to device copy with little involvement of the CPU. (ATA may be able to do this as well, I don't know). Can USB storage do it, if so it will have nothing to do with cache settings or speed. (Note: the buffers and cache that I talk about are not in the storage devices, but the core memory of the computer.) –  richard Sep 19 '12 at 14:31

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