And you need to download a full-size macOS installer for making this disk. Installing macOS without internet requires a bootable USB disk. Having a macOS offline installer or a bootable USB disk is the only solution in such scenarios. But there are situations where you can’t download the OS from the Apple servers. Apple also provides an option for internet recovery of macOS if your computer fails to boot. The online OS update is OK if you have stable broadband connectivity and enough bandwidth. Be it a clean install or upgrade, Mac App store require you connect to the internet to download whole installation files. Customers get Software upgrades for Mac computers through the online app store ever since the release of macOS Lion.
