Assuming you have found a legitimate .dmg file (approx. 5.2 GB), here is how to extract and verify it.
If you are looking for a pre-made raw DMG or ISO file because you cannot use the App Store method, you will likely be looking at Archive.org or reputable tech forums.
Why Archive.org is your best bet: Archive.org hosts digital preservation projects. It is the standard repository for "abandonware" software. You can often find disk images uploaded by the community that are exact replicas of the original installer media. Macos High Sierra 10.13.6 Raw Download
Search Term: macOS High Sierra 10.13.6 Archive.org
Q: Is downloading a RAW macOS High Sierra ISO illegal? A: macOS is free software. Downloading a RAW installer for personal use on Apple hardware is generally considered acceptable, provided you don’t redistribute it for profit. However, using unofficial patchers violates Apple's EULA (but no one has ever been sued for it). Assuming you have found a legitimate
Q: How big is the 10.13.6 RAW download? A: Approximately 5.2 GB for the base installer. The combo update adds another 2 GB.
Q: Can I use this RAW file in VirtualBox?
A: Yes. Convert the .dmg to .iso using Terminal: hdiutil convert /path/to/installer.dmg -format UDTO -o /path/to/highsierra.iso. Cause: Corrupted download or incorrect file extension
Q: Will my 2024 MacBook Air run High Sierra? A: No. Apple Silicon (M1/M2/M3/M4) Macs cannot run Intel versions of macOS. You must use a virtual machine like UTM, but performance will be poor.
Cause: Corrupted download or incorrect file extension.
Fix: Try changing .dmg to .cdr or use hdiutil attach -nomount in Terminal.