Creating the installer will erase all data on the USB drive, so make sure to back up any existing data that may be on the drive. To create a bootable macOS High Sierra USB installer, you need a USB 2.0 or USB 3.0 drive with a capacity of at least 8GB. Therefore, close the installer app by pressing Command-Q on your keyboard. This app is how you would normally upgrade your Mac, but we don’t need to run it now for the purposes of creating our bootable USB installer.
Once it’s done, the High Sierra installer app will automatically launch. The macOS High Sierra download is relatively large at just over 5GB, so it may take some time to download depending on the speed of your Internet connection. When High Sierra is finally released, you’ll be able to find it linked in the sidebar of the Mac App Store homepage.
The first step to create your own bootable macOS High Sierra USB installer is to download the app-based installer Apple provides via the Mac App Store. For the current beta, users will find High Sierra in their Purchased tab after registering their Mac. Step 1: Download macOS High Sierra from the Mac App Store
