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When using several harddrives on an USB 3.0 hub, copying large files between them gives transfer rates far below expectation. My guess is, that the hub introduces contention delays as we see if copying files inside a single harddrive.

For example, I have two drives sporting about 100 MB/s reading and writing, if used solely on the USB 3.0 hub. Copying between them gives varying results of about 8-30 MB/s. The single host connection, which is used two times of course, should sport about 300 MB/s for USB 3.0. So we would expect sustained transfer at 100 MB/s even for two drives.

Are there any adjustments that can be made to the IO scheduler or USB stack, to keep the possible transfer rate?

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  • The phenomenon I tricle for two days now gets a new twist: Using external power, transfer rates attend the expected speeds. While all drives are spinning happy, only one gets the expected transfer speed at a time without external power. There are no hints in dmesg or something. It seems the drives, or the hub exchange information on available power and adjust operation to it. Is there any known feature or reference for such behaviour?
    – dronus
    Aug 16, 2015 at 13:25

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