As far as I know, there are 3 simple ways to download macOS High Sierra installer app for making bootable USB drive. The first is to get a High Sierra compatible Mac. The second is finding a bootable dmg file and burn it to USB. And the last is to reinstall High Sierra from Internal recovery.
Method 1: Get a High Sierra compatible Mac
If you have access to an older Mac that still supports High Sierra, you can download the macOS High Sierra installer directly from the App Store. Once downloaded, it appears as "Install macOS High Sierra.app" in the Applications folder.
How to download and install macOS - Apple Support
From there, you can create a bootable USB using Terminal (createinstallmedia), which is the cleanest and safest method.
sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume
Method 2: Download High Sierra DMG and Burn to USB
This method is popular when you don't have a compatible Mac. You download a prebuilt High Sierra DMG file, then burn it to a USB drive using tools such as SysGeeker UltraDMG.
It is a desktop utility designed to help users download macOS dmg files and create bootable USB drives on both Windows and Mac. It supports common disk image formats like DMG, ISO, and IMG, and simplifies the process into a few clicks, making it especially useful when your Mac won't boot or when you need to install macOS (like High Sierra) on another device.
Step 1. Download and install UltraDMG on a working Windows or Mac computer, then launch the program.
Step 2. Go to the "Download macOS" section, choose the correct macOS version, and download the High Sierra DMG installer file directly to your computer.
Step 3. Insert a USB drive (at least 16GB), switch to the "Burn" feature, load the downloaded DMG file, select the USB drive, and click Burn to start creating the bootable installer.
Step 4. After the process completes, plug the USB into your Mac, boot from it (Option key on Intel or Power button on Apple Silicon), and follow the on-screen instructions to install macOS.
UltraDMG is a beginner-friendly tool that combines macOS downloading and USB creation into a simple workflow, avoiding complex Terminal commands and making it easy to build a macOS bootable installer in just a few steps. It’s a convenient option when you don’t have access to a compatible Mac, though you should still ensure the downloaded installer source is reliable before using it.
Method 3: Download and Reinstall High Sierra from Internet Recovery
This method uses Apple's built-in recovery system.
Internet Recovery is a native feature on Mac computers that allows you to reinstall macOS directly from Apple’s servers, even if the internal drive is empty or the system is corrupted. Instead of relying on a local recovery partition or USB installer, it downloads the compatible macOS version (often macOS High Sierra for older Macs) over the internet and installs it cleanly.
Use macOS Recovery on an Intel-based Mac - Apple Support
Step 1. Shut down your Mac completely.
Step 2. Turn it on and immediately press and hold Option + Command + R until you see a spinning globe (this starts Internet Recovery).
Step 3. Connect to a Wi-Fi network when prompted and wait for recovery tools to load.
Step 4. (Optional) Open Disk Utility, select your startup disk (e.g., Macintosh HD), and erase it if you want a clean installation.
Step 5. Return to the main menu and choose Reinstall macOS, then follow the on-screen instructions.
Step 6. Wait for macOS High Sierra to download and install (this may take time depending on your internet speed).
Internet Recovery is the easiest official way to reinstall macOS High Sierra without needing a USB installer or another Mac. It works entirely online, making it ideal for fixing boot issues or restoring a system, but it requires a stable internet connection and doesn’t give you full control over installer files like a bootable USB would.
