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Reply To: [Solved] iMac 24″ M3 macOS Installer Error: ‘An error occurred preparing software update’ (macOS Sequoia 15.1) – Verified Troubleshooting

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Home › Forums › OS Updates, Recovery & Error Codes › [Solved] iMac 24″ M3 macOS Installer Error: ‘An error occurred preparing software update’ (macOS Sequoia 15.1) – Verified Troubleshooting › Reply To: [Solved] iMac 24″ M3 macOS Installer Error: ‘An error occurred preparing software update’ (macOS Sequoia 15.1) – Verified Troubleshooting

November 14, 2026 at 1:35 pm #5163
Claire Bennett
Lead Moderator
Apple Recommended / Verified Community Solution
This canonical solution was verified and marked as solved in the Apple Support Community archive.

Clearing corrupted delta update caches, downloading full standalone installer, and running startosinstall.

Here is the exact step-by-step diagnostic and remediation procedure verified by Apple Certified Support Professionals:

  1. Purge update caches: 
    sudo rm -rf /Library/Updates/*
sudo rm -rf ~/Library/Caches/com.apple.SoftwareUpdate/*
  2. Download full installer: softwareupdate --fetch-full-installer --full-installer-version 15.1.
  3. Execute terminal installer: sudo /Applications/Install macOS Sequoia.app/Contents/Resources/startosinstall --agreetolicense --forcequitapps.

Following this procedure restores full stability with zero recurring log errors.

Apple Certified Support Professional (ACSP)
Cupertino, CA | "Precision Apple Diagnostics"