HOW TO Clean install macOS

1 Back up your Mac

Before reformatting your Mac’s boot drive, you should back it up with Time Machine, just in case anything goes wrong. Plug in an external drive, go to System Preferences > Time Machine and back up in the usual way.

2 Back up data

Check your Mac for documents, photos, videos and other files you don’t wish to lose, and back them up too. If your Mac was languishing in an attic or cupboard there probably won’t be any, but check anyway.

3 Download High Sierra

Head to https://apple.co/3v23zOI, which is the Mac App Store listing for High Sierra. Click on View in the Mac App Store, then click the Download button to download the High Sierra installer. Quit it when it opens.

4 Make an install drive

You can reinstall High Sierra over the internet, but that can prove unreliable, so we’re making a bootable install drive. Go to https://diskmakerx.com/download and download DiskMaker X 7 (for High Sierra). Install it on your Mac.

5 Launch X 7

Plug in an empty USB stick or drive of at least 8GB in size. Launch DiskMaker X 7; if you’re offered a newer version, decline. It’s DiskMaker X 7 you need. The installer is found and located on your Mac. Click "Use this copy".

6 Choose the drive

When invited to do so, choose the USB drive you plugged in. It will be erased, so make sure that there’s nothing on it you need. Follow the on–screen instructions, typing your admin password when asked to do so.

7 Wipe the drive

The installer is created. It might take some time; you get a notification when it’s finished. Shut down your Mac, then restart holding Cmd+R to boot in recovery. Open Disk Utility, and erase your main internal drive.

8 Install High Sierra

Click the Apple icon in the top–left corner, and select Startup Disk. Choose the installer drive you made earlier from the menu, and then follow the on–screen instructions to reinstall macOS High Sierra on your Mac.