hdiutil attach /tmp/HighSierra.dmg -noverify -mountpoint /Volumes/HighSierraInstaller Use code with caution.
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Once you have installed High Sierra using your DMG:
A: Download size is around 5–6 GB. Installation from a USB drive typically takes 20–40 minutes, depending on your Mac’s hardware and whether you choose to upgrade or clean install.
Insert your newly created bootable USB installer into a USB port. Turn on the Mac while holding down the key.