Now go into Settings and turn OFF "Background Rendering" and NEVER turn it back on, ever.
Those background renders start to eat up TONS of space (Apple, PLEASE do something about this serious issue) and can get corrupted and cause problems. More problems than they're worth. And most of the time, rendering a timeline is unnecessary.
A system drive (for any operating system) needs at least 15% of its total capacity left as free space at all time. This lets the OS write and read temporary working files you and I never see, hear, smell, or know about. If there's not enough space for these operations to take place seamlessly, all sorts of weird behaviors start to happen. Don't let FCPX eat up all your drive space!