Fix “This copy of the installer is damaged”

Why old macOS installers fail with expired-certificate errors, and how to fix them.

Older macOS installers are signed with Apple certificates that have since expired. If you run one today, you might see:

• “This copy of the Install macOS … application is damaged, and can't be used to install macOS.”
• “An error occurred while preparing the installation.”

The installer is fine. The Mac is just checking the certificate against today's date.

Fix 1: Download a fresh copy

Apple has re-signed most of its older installers. Installers downloaded from the links on this site (the Mac App Store, Apple's CDN or softwareupdate) usually work. Delete any copy you've had for years and download again.

Fix 2: Set the clock back (offline installs)

If you're installing from an old USB drive, boot into the installer, open Utilities › Terminal, disconnect from the internet, and set the date to around the version's release year. The format is MMDDhhmmYY. For example, 1 January 2018:

Quit Terminal and run the installer again. The clock corrects itself when the Mac goes back online.

Old Macs can't open modern websites

Very old versions of Safari can't load many HTTPS sites, including some Apple download pages. Download the installer on a newer computer, then copy it across with a USB drive.