When you visit Apple’s official website today, the link for High Sierra usually redirects to a tiny InstallAssistant.dmg that is roughly . This is a web installer. Once launched, it tries to download 5GB of data from Apple’s servers. The problem? Apple’s certificate for High Sierra frequently expires, causing the download to fail with a "Resource temporarily unavailable" error.

macOS High Sierra 10.13.6 full installer - Apple Communities

(Replace MyUSB with your USB’s name.)

Downloading MacOS High Sierra 10.13.6 is relatively straightforward. Here are the steps:

After you finally secure the file, do not just double-click it. To truly future-proof your system, create a bootable USB installer. This allows you to wipe your hard drive completely and install a fresh OS.

Right-click the installer → Get Info → Ensure size is 5.19 GB or larger.