We’ve seen a few cases recently where a user has unexpected lost all data, and been left with a disk named OS X Base System. Inside the disk is a file named Install macOS Sierra or Install macOS High Sierra, and a few other system files.
The OS X Base System is usually part of a macOS installer or update, so it’s unclear how disks ended up getting replaced with this.
If this has happened to you, we’d love to hear more about how.
We can usually recover these disks, but it’s really important that you stop using the disk as soon as possible. If you install anything to the disk you could risk losing all the data permanently.
We’ve still got more of these to investigate so we’ll update the post when we’ve learned more.
Update: Could be related to Internet Recovery?