When you mirror files, all of your My Drive files are stored in the cloud and on your computer, which takes up hard drive space. All of your files will be accessible at any time, even when you don’t have an internet connection or the Drive for desktop app running. Files can be accessed through a folder on your computer.
When you stream files, your My Drive files are stored in the cloud. Hard drive space is only used when you open files and make them available offline. Files that are not available offline can only be accessed when you’re connected to the internet. All files, including those available offline, can be accessed only when the app is running. Files can be accessed through a mounted drive on your computer.
Important: These options don’t apply to any folders on your computer that you sync with Google Drive. Folders on your computer sync to the Computers tab in Google Drive.