Digging around, I found that the Fusion 360 Streamer.Log was 31GB, yeouch!
According to the fine folks at AutoDesk this is a known issue and they will have a fix for it soon. In the interim, you can fix it to get the disk space back.
To fix it:
- Use Task Manager to kill streamer.exe
- C:\Users\<your username here>\AppData\Local\Autodesk\autodesk.webdeploy.streamer.log