This typically happens when trying to reinstall High Sierra via Internet Recovery. The URL your Mac attempts to contact is often an https address, while Apple hosts the file on an http server. The most reliable fix is to bypass Internet Recovery entirely by creating the bootable USB installer as described above.
Apple provides the installer as a hidden download on their support servers. While the file typically downloads as a .app (Install macOS High Sierra.app) inside a disk image, users often look for the raw DMG for virtualization or archive purposes.
Connect a USB flash drive with a capacity of to your Mac.
To use your new bootable drive, shut down the target Mac completely. Insert the USB drive, hold down the key, and press the Power button . Keep holding the Option key until the startup manager screen appears, then select Install macOS High Sierra from the boot options list. Critical Troubleshooting & Safety Tips
Even in 2026, Apple maintains a hidden support page for legacy installers. This is the safest method.
Apple provides direct DMG downloads for legacy systems that cannot access the App Store. Apple Support Direct Download: Apple Support Download page and look for the High Sierra link. Install the Installer: Double-click the downloaded file to open it. Inside, run the file to install the installer app into your Applications folder. Apple Support 4. Third-Party Alternative: Mist
Insert your newly created bootable USB drive into a USB port.
Once you have the installer in your Applications folder, you can turn a USB flash drive into a bootable installer. This is highly useful for clean installations or troubleshooting broken systems. Prerequisites: A USB flash drive with at least of storage.
After the installation is complete, you'll be greeted by the Setup Assistant. Follow the on-screen instructions to:
You will also need at least to perform the upgrade.
Note: Avoid downloading DMG files from third-party torrent sites or unverified file-sharing blogs, as they may contain malware or modified system files. How to Create a Bootable USB Installer from the Download