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When I connect my SanDisk USB flash drive to USB 2.0 port and write 102MB of data to it, then the data transfer speed is 9.2MB/s:

# time dd if=/dev/zero of=/dev/sdb bs=512 count=200000
200000+0 records in
200000+0 records out
102400000 bytes (102 MB, 98 MiB) copied, 11,1333 s, 9,2 MB/s

real    0m11.134s
user    0m0.000s
sys 0m0.244s
# 

When I connect this same USB flash drive to a USB 3.0 port in this same PC, then the data transfer speed is 15.4MB/s:

# time dd if=/dev/zero of=/dev/sdb bs=512 count=200000
200000+0 records in
200000+0 records out
102400000 bytes (102 MB, 98 MiB) copied, 6,63853 s, 15,4 MB/s

real    0m6.639s
user    0m0.028s
sys 0m0.180s
# 

What might cause this? USB 2.0 theoretical maximum transfer speed should be 60MB/s, i.e I would expect the same ~15MB/s transfer speed both in high-speed and SuperSpeed modes.

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Check user reviews on Amazon. This flash drive is reported to be slow. The specs never give a hard figure for write speed. The USB3 connector or USB3-compliance has never been a guarantee of speed.

You may get different results if:

  • you format a filesystem on it and write to files instead of the raw device
  • you test with a hundred 1MB files instead of one single 100MB file.

Also /dev/urandom is a better emulation of the typical files you write on these devices than /dev/zero. But don't use /dev/random as it is way to slow and would be the bottle neck.

  • Point of my question was that write speed to USB flash-drive was faster in case of USB 3.0 port than in case of USB 2.0 port while I would have expected the same write speed both in case of USB 2.0 and USB 3.0. – Martin Jun 9 '16 at 18:03
  • @martin Theoretical transfer rates are never reached... USB2 is 480Mbits/s, so you should be able to achieve write rates of 60MBytes/s, but I have never seen a USB2 external disk go faster than 20MBs, while as an internal disk you can achieve 60MBs. Given the low write speed of your key, the USB2 bus is less of a limiting factor and you see less difference than with a better device (20MBs and 90MBs on my USB hard disk) – xenoid Jun 9 '16 at 21:06

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