Download High Sierra from the the App Store and place a formatted USB drive named ‘Untitled’ and fire this command into the terminal. Enter your system password and let it run for several minutes.
sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia —volume /Volumes/Untitled —applicationpath /Applications/Install\ macOS\ High\ Sierra.app
Tip: Press Ctrl+T to check progression.
Thanks to: Macworld
How to install OS X | macOS
1. Create a usb install drive using my previous post.
2. To start from a USB storage device connect the device, restart, and immediately press and hold the Option key to access Startup Manager. Apple Support:
Startup key combinations for Mac
Starting from an external USB storage device (Intel-based Macs)
3. Select the USB device and follow the installation wizard.
4. Organize the partition(s) with Disk Utility and install OS X.
5. Wait for about 30 minutes to complete and follow the wizard to finalize.
Enjoy your fresh installation!
hold down the option key to access Startup Manager
