Preparing For Installation Mac



Hello, I ran into the same issue while trying to factory-reset a Mac Mini 2012 with MacOS High Sierra from a USB key. The Mac Mini had a custom Fusion Drive with High Sierra on it, and was previously formatted with APFS. Since moving to Snow Leopard, I've noticed an innumerable number of times where software installations would get stalled at the 'Preparing.' If you've noticed it too, chances are you're an iPhone developer, too. It turns out the simple fix for this problem is to quit the iPhone Simulator, and then try the installation again. Cancel and restart. If you have stuck downloads in the Mac App Store or updates that won’t start. Oct 16, 2019 I have erased the hard drive on my iMac (Early 2008) and am trying to re-install El Capitan (10.11.6) that was running on it. It was acting up so decided a re-install was the best way to go. Followed the instructions and all went well until it it restarted. Jul 16, 2020 Whenever you choose to update an iOS or iPadOS device, it downloads the update file from Apple’s servers first and then proceeds with the installation. Once the download is complete, the iPhone starts preparing for the update. Occasionally, this process gets stuck, and in such a situation you’ll have to redo the update process.


Installing Mac OS X on iMac's blank formatted hard drive. Installation runs for 5-10 minutes and then fails with following error:

An error occurred while preparing the installation. Try running this application again.

Tried installing OS X El Capitan (10.11) and OS X Sierra (10.12) - the same error occurs in both cases.


Preparing For Installation Machine

Issue was resolved by fixing incorrect time on the iMac and then clearing NVRAM.

Fix incorrect time

  • Boot from Mac OS X installation disk / USB.
  • Go to Utilities > Terminal.
  • Type date to check the current date and time.
  • If it's incorrect, run following to fix the date and time: date mmddHHMMyyyy
    Where: mm - month, dd - day, HH - hour, MM - minute, yyyy - year


  • Shut down the Mac
  • Start the Mac and as soon as you hear startup chime press and hold keys Command + Option + P + R.
  • Keep holding the keys until Mac reboots and you hear another startup chime.
Preparing For Installation Mac

iMac 21.5' (2009)
Mac OS X El Capitan (10.11)
Mac OS X Sierra (10.12)

View the discussion thread.


Seagate hard drive mac.

I ran into the same issue while trying to factory-reset a Mac Mini 2012 with MacOS High Sierra from a USB key.

The Mac Mini had a custom Fusion Drive with High Sierra on it, and was previously formatted with APFS. The Fusion volume had been erased and reformatted with Journaled HFS+.

(@Apple technical staff: I hope this info helps you to improve the great products you are working on)

Preparing for installation machine

Just as in your case, the installation process went smoothly, up to the point where the installation disk contents where copied to the target volume, and Kaboom! : I got the screen as shown in Unable to install High Sierra - 'an error occurred while preparing the installation'

As you may know, APFS on Fusion drives is currently at Beta stage, so glitches are to be expected.


As stated before, however, the Fusion drive had previously been erased with Disk Utility and reformatted as 'Journaled MacOS Extended'.

Here is how I solved the issue:

  • The issue came from the inability for the installer to unmount the Fusion drive before rebooting. I figured this out in Disk Utility as it could not unmount the drive to 'repair' it.
  • So I used the instructions in the second part of this Apple documentation: APFS Fusion
    • Open a Terminal window (in the Installer launch screen, it is under the 'Utilities' menu item.
    • Look for the Unix names of your drives:
      • #> diskutil list internal
      • take note of which drive is the SSD, which is the HDD. In my case, it was as in the example from Apple: SSD was disk0, HDD was disk1.
    • Force-unmount your target logical volume:

      #> diskutil unmount force disk2s1

    • Separately re-format each Drive of the previously-unmounted Fusion volume:
      • #> diskutil eraseDisk JHFS+ SSD disk0
      • #> diskutil eraseDisk JHFS+ HDD disk1
    • Recreate a Fusion Drive:

      #> diskutil cs create 'Macintosh HD' disk0 disk1

    • Recreate a logical volume on it:

      diskutil cs createVolume 'Macintosh HD' JHFS+ 'Macintosh HD' 100%

    • Quit the Terminal application. You'll be taken back to the installer.
  • Retry to install High Sierra. In my case the issue was completely solved.

Preparing For Installation Mac Os

Preparing For Installation Mac

Hope this helps, good luck!