所以 Apple 也開始玩自動下載新版作業系統這招了...

在「Apple starts downloading MacOS Sierra automatically to your MacBook — Here's How to Stop It」這邊看到 Apple 會自動下載新版作業系統,大約吃 5GB 的流量愈空間:

If you have automatic downloads enabled on your Mac, a large file of around 5GB will mysteriously be downloaded to your computer in the background, using your Internet bandwidth for unrequested files.

關掉的方式在這邊:

To disable the feature, you can head on to System Preferences → App Store → Automatically check for updates and then uncheck "Download newly available updates in the background."