What are you currently using to download the file (macOS or Windows)?

Apple distributes the OS as an .app file rather than a .dmg . If you need an actual DMG image for virtual machines or deployment, you can build one yourself using Terminal: Open .

Click . This actions launches the Software Update panel in your System Preferences.

If the Terminal command fails (Apple occasionally deprecates older OS links), two generally safe sources exist:

If you choose to use a third-party link because you are downloading from a Windows PC or an unsupported computer, protect your data by following these rules:

A preserved image of the installer is available via the Internet Archive macOS Catalina Library . Be sure to verify the hash of the file if possible to ensure it has not been modified. How to Create a Bootable USB Installer using your DMG

Fix Macs experiencing boot loops or corrupted recovery partitions.

Now that you have your installation media—whether a bootable USB or the installer in your Applications folder—you are ready to proceed.