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So I have an external SSD connected with a SATA to USB 3.0 adapter to my retina MacBook Pro. The wire supports speeds up to 6 Gbps. I could write to the SSD itself at such a high speed.

I created two partitions on the SSD – one for the TM backups, and one for storing files. However, whenever I try to transfer files to it now, the maximum speed it will reach is 20 MBps.

Is there a reason that it has become so much slower? Is this an inevitable consequence of having a TM drive, or is this an unusual circumstance which should not be occurring?

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  • 20 MB/s looks like USB 3.0 is falling back to USB 2.0. Please check the speeds of the various devices in the USB device tree in System Information.app
    – klanomath
    Commented Mar 25, 2017 at 17:01
  • What speed do you get copying a 5 GB file like the macOS Sierra installer from the internal drive to external SSD? If 20 then @klanomath nailed the diagnosis.
    – bmike ♦
    Commented Mar 25, 2017 at 17:52
  • @klanomath Well, I just tried it again, and it seems as though the read/write speeds are 40 MBps, not 20 (I'm not sure where I got the number 20 from). The USB device tree says that it is connected to the USB 3.0 Bus but states Speed: Up to 480 Mb/sec, so it seems like you're right.
    – Oion Akif
    Commented Mar 25, 2017 at 18:11
  • @SkeletonBow I usually get 20-30 MB/s transferring files via USB 2.0 busses but never 40-48 MB/s - so interpreted the 20 MB/s as a "magnitude" slower compared to USB3>HDD ~150 MB/s and USB3>SSD ~250-300 MB/s ;-).
    – klanomath
    Commented Mar 25, 2017 at 18:20
  • No no, don't worry! You've definitely found the problem – it should show 6 Gbps, not 480 Mbps. Now the only question is why. I really don't know what to search up, either.
    – Oion Akif
    Commented Mar 25, 2017 at 20:35

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