(The drive should be at least the same size as your Mac drive.)
Here you have two options to backup Mac data before macOS 10.13 update: Apply Time Machine or Mac backup software for help.
Back up Mac computer data before macOS 10.13 update Here Part 1, we’ll show you how to upgrade to macOS 10.13 High Sierra without losing any data on MacBook Pro/MacBook Air/iMac etc.