After downloading the DMG file, verify its integrity to ensure it hasn't been tampered with:
Downloading the macOS High Sierra (10.13) installer as a DMG file is a bit different than newer versions because Apple provides it through the App Store rather than a direct web-download link for the full OS image . YouTube +1 1. Official Download via Mac App Store The most reliable way to get the installer is through the official App Store link . Note that High Sierra will not show up in a standard search on newer macOS versions; you must use a direct link. YouTube +2 Requirements
To create a bootable installer from the DMG file:
Users specifically searching for a usually have a specific use case, such as creating a bootable USB drive for a VM (Virtual Machine) or legacy hardware repair.
Medium. While High Sierra is old, downloading a pre-packaged DMG from a forum or file-hosting site exposes the user to potential integrity tampering. The safest route is always to convert the official App Store download.
: Use Disk Utility to format the drive as "Mac OS Extended (Journaled)" with a "GUID Partition Map."
: Ensure it has at least 16GB of space and backup any data on it.