My iMac's hard disk has been slow from the start. But recently it is becoming unbearable. Disk read and write speeds are consistently below 50 MB per second, without any applications producing any CPU load.

I do see that mds uses lots of disk I/O. Could this be the problem? If so, what can I do to alleviate it? I tried turning off the indexing with sudo mdutil -a -i off, but it did not help much.

How else can I debug this?

mds shows huge disk I/O

Blackmagic Disk Speed shows R/W rates of less than 50 MB/sec

  • Let’s see how much space is currently available on the boot volume. Open Terminal and post the output of the following df -h | awk 'NR <= 2'
    – njboot
    Commented Apr 18, 2016 at 3:43
  • Back when I had the problem I had more than 200GB free space on my volume.
    – Arne
    Commented Apr 18, 2016 at 8:31

1 Answer 1


To finish this question: I had opened a bug report with Apple. They determined that a botched Aperture photo library migration lead to excessive disk I/O. In the end, I reformatted the whole Mac. Plus to make the machine more useable, I attached a 256 GB Thunderbolt SSD and created a FusionDrive, as per this blog (in german). There are lots of howtos explaining how to create a FusionDrive. This really gave the machine an incredible performance boost.

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