Your old installers do not work. Yes, the ones you saved for when you would need to downgrade purchases to an older OS or re-install on top of a dysfunctional (older) OS. You get an error message. The problem is most likely not with your installer, but that Apple stopped “signing” their installers. Which seems to be an off-line proces…
You can try to set your computer’s date to a date within a year of the OS’s initial release date (while turning off your internet connection) during install or creation of a USB boot stick.
If that does not work, you need to re-download the needed installers. Pay attention to the warnings on this page about carefully marking the various re-downloaded installers, as they will arrive on you computer indistinguishable from each other!
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Then follow the download links from below:
When trying to download various versions of macOS (including older ones), I noticed, that it did not always work as expected. So i took some notes and put together this guide for reference.
Via App Store links or Apple direct download links
You can download High Sierra, Mojave and Catalina via App Store links:
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How to upgrade to macOS Catalina – Apple Support
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App store link: Get macOS Catalina
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How to upgrade to macOS Mojave – Apple Support
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App store link: Get macOS Mojave
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How to upgrade to macOS High Sierra – Apple Support
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App store link: Get macOS High Sierra
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For El Capitan and macOS Sierra Apple provides the direct download links instead of the App Store links in its own documentation. The App Store links will only work on High Sierra and below, as Mojave and Catalina will refuse to download these older versions.
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How to upgrade to macOS Sierra – Apple Support
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App store link: Get macOS Sierra
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Direct download link: Download macOS Sierra
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the installer will copy Install macOS Sierra into /Applications
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How to upgrade to OS X El Capitan – Apple Support
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App store link: Get OS X El Capitan
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Direct download link: Download OS X El Capitan
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the installer will copy Install OS X El Capitan into /Applications
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How to upgrade to OS X Yosemite – Apple Support
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Direct download link: Download OS X Yosemite
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the installer will copy Install OS X Yosemite into /Applications
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For even older versions try here: How can I download an older version of OS X/macOS? – Ask Different
Some of the problems relating to downloading macOS have been covered in this article: How to download macOS Catalina, Mojave or High Sierra Full Installers
gibMacOS for downloading macOS Recovery Image or creating a Full Installer
The gibMacOS python script runs on Windows, Linux or macOS and is used in this Guide: Downloading the Recovery HD image – /r/Hackintosh macOS Internet Install – For example to get the Yosemite Recovery Image run in the Terminal ./gibMacOS.command --recovery -v 10.10 -m 10.10
(replace 10.10 with the version you need: 10.09 to 10.15)
gibMacOS has a script to create the whole installer from the parts downloaded. You can only get the Full Installer of the latest supported OSes by Apple (like High Sierra, Mojave and Catalina).
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get the gibMacOS script
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run the script in Terminal
./gibMacOS.command
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select the appropriate entry
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let it download and wait (when finished type ‘q’ to quit)
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run
./BuildmacOSInstallApp.command
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drag the appropriate download folder onto the Terminal window (press enter and wait)
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the “Install macOS {MacOS Version}.app” will be in the same folder
More details here: Preparing macOS for Installation. Thanks to midi1996 and fewtarius who provided additional instructions for gibMacOS
Dosdude1 Patchers
Each GUI App has a macOS downloader included: dosdude1. For a hackintosh, you do not need to use any of the other features. The App downloads the full macOS installer from swcdn.apple.com (checked with my firewall)
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macOS Catalina Patcher (Download macOS Catalina Patcher 1.3.0)
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Download macOS: Continue > Continue > Download a Copy
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macOS Mojave Patcher (Download macOS Mojave Patcher 1.3.7)
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Download macOS: Menu > Tools > Download macOS Mojave…
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macOS High Sierra Patcher (Download macOS High Sierra Patcher 2.7.0)
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Download macOS: Menu > Tools > Download macOS High Sierra…
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Edit: Added info for Yosemite and for older Versions