I'm copying about 4TB of data from one hdd to another and noticing abysmal speeds.

I have two external SATA 3 hdds plugged into USB 3.0 docks and connected to USB3.0 ports on Macbook Pro. MBP was idle during the copy.

Obviously, the speed is limited by disk mechanics - according to manufacturer the read/write speed is 150MB/sec. So theoretically, 1TB should take 2hrs to copy over. I'm seeing times almost double that, which means I'm getting about 80-90MB/sec.

How can I raise my transfer speed close to max speed of 150MB/sec? 150MB/s is way below the capabilities of SATA3 or USB 3.

  • 1
    If your speed is 150MB/sec then it will just take 8-10 sec to copy 1 GB Data not 2 Hrs – Ali786 Aug 5 '14 at 16:38
  • > copying about 4TB of data. 80-90 MB/s over a large set of data is quite good in my opinion. – Hannu Aug 5 '14 at 16:58
  • Ensure both drives aren't sharing a single internal USB hub. – Ƭᴇcʜιᴇ007 Aug 5 '14 at 17:06
  • If you can, retest with a powered USB 3 hub. Power may be the performance-limiting factor here. – David Schwartz Aug 5 '14 at 17:20
  • For what it is worth: Practical testing just the other night - Asus HERO VI USB 3.0, i7 4770, 32GB RAM - cheap USB 3.0 to IDE 3.5" (self powered) directly connected to the MB port, an 320GB ST320620A drive. The very first GB may achieve as much as 200MB/s, then it tops out at 80 MB/s which slowly fades to 55-60MB/s as the disk comes close to full. Test with dd if=/dev/zero bs=8k of=ON-DISK – Hannu Aug 5 '14 at 18:37

There are a couple of factors which could be at play here, but you've probably just reached the max speed they can support. The docks you are using, although USB3, might not be able to support the same throughput as your hard drives, and therefore slowing down the copy process. It could also be to do with the formatting of the hard drives themselves. Also an app on your MBP could be reading/writing to the hard disk during the copy process, slowing it down.


You Don't Need to do anything to raise the transfer speed the transfer speed is at its maximum speed, but the time that slowing down your system is because of seek and latency.

For finding, seeking and latency will take time and slow down your speed if you have small files then it will take much more time than transferring big files.

You can use file copying tools to increase the speed as they use some internal algorithms to speed up the transfer speed like terracopy etc.

see this question also. Slow internal disc to disc file transfer speed

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