Odd performance drop with external SSD on new iMac Pro

Discussion in 'Mac Pro' started by MarckyG, Dec 5, 2018.

  1. MarckyG, Dec 5, 2018
    Last edited: Dec 5, 2018

    MarckyG macrumors member


    Jan 15, 2014
    Hello professionals :D

    I'm a proud owner of an iMac Pro for a few days now.
    I'm coming from an iMac 2013 with Thunderbolt 1 and no retina display.

    I observed an oddity with external drives and wanted to ask for advice...
    It's with the SanDisk Extreme 900 (a 1,92TB SSD in Raid0).
    While on my old Mac with Sierra and HFS+ the drive had normal speeds over USB3.0: around 420MB/s write and 450MB/s read.
    Now with my new iMac, I was eager to test the actual speeds this drive can do with a USB-C Thunderbolt 3 connector...
    250MB/s write and 850MB/s read... The USB 3.1 Gen2 protocol definitely let's the read speeds go to their full potential, but the write speeds suffer? And it's the same with USB-A cable or USB-C 3.1 Gen2 cable...
    The read speeds go to around 400MB and after a second they drop to 300 and then stay at around 250MB/s...
    I tested with Blackmagic Disk Speed and a file copying while watching the timer confirmed that...
    Going back to my old iMac with HFS+ the write speeds are again better over the USB-A cable...
    Does anyone have an idea? Is APFS at fault?
  2. mattferg macrumors 6502

    May 27, 2013
    This isn't the iMac Pro forum, I suggest you post there!
  3. AidenShaw macrumors P6


    Feb 8, 2003
    The Peninsula
  4. MarckyG thread starter macrumors member


    Jan 15, 2014
    Oh! Sorry, thanks for correcting me! I'll post there!

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