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I'm having difficulty installing the latest update on my Mac Mini from 2011 (macmini5,3).

The current pending update is "Security Update 2019-003-10.13.6." When I attempt to install the update:

  • It downloads the update.
  • It asks to reboot.
  • It gets ~5% through a progress bar then pops up an error message: The update was not installed successfully. You can try updating again in the App Store after restarting.

I have attempted in order:

  • Manual download and install of the update.
  • Fixing disk issues in repair mode (none were found).
  • Installing the update in Safe Mode.
  • Installing the update in Repair Mode.
  • Reinstalling macOS.
  • All of the above again, with a support tech in chat. After which he stated he couldn't help me and I should go to an Apple Store for support.

The issue that triggered the wave of triage, is that Dropbox needs to be manually upgraded by downloading a new version. The download is delivered as a .dmg. It turns out that in this state, the os is unable to open a .dmg and fails with "detaching because no mountable filesystems." This same message is displayed for both the dropbox updates as well as the apple system update (SecUpd2019-003HighSierra.dmg).

I "feel" (hahaha, no data to support this) like the format for .dmg file has changed. And apple failed to release an update to this version of macos such that the format is no longer readable.

Update: I was able to resolve by wiping the disk and installing from scratch.

  • I have a similar issue with this security update, but not identical, on a MacBook Pro 2015 with 10.13.6. Symptoms are that the progress bar on reboot gets virtually to end, then resets to zero, then I gave up and pressed power button. – RichVel Jun 1 at 14:37
  • I later solved this by rebooting into Safe Mode (hold down Shift while rebooting) to install the update. See this comprehensive answer on solving difficulties with applying macOS updates. – RichVel Aug 22 at 5:39
  • I updated my answer to include my resolution. The approach of installing from safe mode didn't work for me. I'm pretty impressed by the variety of scenarios that do + do not work with this issue. – Raymond Kroeker Aug 23 at 17:50

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