Apple occasionally hosts older installers behind a login wall. Try this direct link (copy into Safari):
https://support.apple.com/en-us/HT206886
Note: This page currently redirects to newer macOS versions. If you do not see Yosemite, use Method 2.
Because Apple’s official certificates for Yosemite expired in 2019, even the official installer requires a date trick to run. The largest verified repository for the Yosemite 10.10.5 DMG is the Internet Archive.
The verified link (no torrent required): mac os x yosemite 10.10 5 download dmg
File details to verify before downloading:
Why use this? Unlike torrent sites, Archive.org maintains a historic software library. The DMG hosted here is unmodified from the original Apple InstallESD.dmg.
If you have access to any functional Mac running 10.9 through 12.x, you can legally download the Yosemite 10.10.5 installer directly from Apple’s content delivery network.
Step 1: Open Terminal (Finder > Applications > Utilities > Terminal). Apple occasionally hosts older installers behind a login
Step 2: Paste the following command exactly as written. This command instructs Apple’s software update catalog to download the final version of Yosemite.
softwareupdate --fetch-full-installer --full-installer-version 10.10.5
Note: On some newer versions of macOS (Ventura/Sonoma), softwareupdate only backs up to Sierra (10.12). If the above fails, use this legacy tool:
sudo /usr/bin/installinstallmacos.py
(You must first download this Python script from GitHub – Grahampugh’s repo – though the first command is preferred.)
Step 3: Once downloaded, the file will appear in your Applications folder as Install OS X Yosemite.app. This is not a DMG yet. File details to verify before downloading:
Step 4: To convert the .app into a mac os x yosemite 10.10 5 download dmg for backup, right-click the app and select "Show Package Contents." Navigate to Contents/SharedSupport/ and locate InstallESD.dmg. Copy that file. That is your pure DMG image.
(If you already have the App Store “Install OS X Yosemite.app” skip to the Terminal createinstallmedia step.)
Convert to an installer app (if you have the Yosemite .dmg containing the Install OS X Yosemite.app):
Prepare the USB drive (will be erased):
Create the bootable USB using Terminal (assumes installer is in /Applications and USB named "Untitled"):
Boot from USB and install: