If you are installing this over an existing system, or an external hard drive to back up your files. Downgrading or reinstalling an OS wipes the disk.
Boot and install
Shut down your Mac, then turn it on while holding the Option (Alt) key. Select the USB installer. download+macos+high+sierra+10136+dmg+mac+install
Grab the .dmg or .app installer from one of the trusted sources listed above. Step-by-Step: Creating a Bootable USB Installer
Check if your can handle a newer OS instead. If you are installing this over an existing
Run this command in Terminal: sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume
Upgrading older Macs directly to newer OS versions without stepping through High Sierra first can cause firmware conflicts. Select the USB installer
Finding a legitimate, safe source to can be frustrating due to Apple’s deprecation of older software links. However, by using the official softwareupdate Terminal command or the hidden App Store URL, you can still obtain the final build of High Sierra (10.13.6 / 17G66) without resorting to piracy.
If you're downloading the .dmg file from a trusted online source, make sure to verify the file's integrity using checksums (e.g., SHA-256).
Set the Format type to (or Mac OS Extended Journaled if you are using an older mechanical hard drive instead of an SSD).
Avoid third-party torrent sites or untrusted download blogs, as they frequently bundle malware or modified system files with the operating system. Always use official Apple sources or verified delivery networks. Method A: The Mac App Store (Recommended)