What is this Google Backup and Sync "Back up your files" dialog asking?

Google Drive recently became Google Backup and Sync, and this dialog started occasionally popping out of its menu bar item: I don't have a "Boot OS X" folder that I'm aware of, in Google Drive or locally. When I look at the settings for Google Backup and Sync, I don't see anything about this. I haven't touched the settings since it was Google Drive, so it's set to sync my Google Drive and nothing more. I can't tell what the files are from the little generic icons, and I can't figure out how to get a list. It's also not clear what device it's talking about, or what choice it's going to remember (since the options are essentially "yes" or "later"). Does anyone know what this dialog is about and what I might have done to cause it?