Why is my Hard-Drive filling up quickly?
A few possibilities:

You're running into memory pressure and are creating a lot of swapfiles. This scenario is likely because the 2017 MacBook Air only has 8 GB RAM, which is not a lot by today's standards and definitely insufficient for any video editing in Final Cut Pro. You can check if this is the case by running /Applications/Utilities/Activity Monitor.app, clicking on the Memory tab, and looking at the Swap Used field at the bottom. If you're seeing many gigabytes, reboot your system. Note that this situation may be unavoidable if you try to do video editing on such a low-end machine.

A process is creating a lot of cache or tmp files. A reboot may clear these.

There are background downloads occurring, possibly for OS updates or possibly for other apps you have installed. A reboot probably would not clear these.


In short, try rebooting. Before you do, though, it would be wise to Get Info on your hard drive in the Finder and see how much space is available, and then compare it after the reboot. If you find yourself still low on space, you'll need to start diving into subdirectories to see where the space is being used. There are also third-party tools that can help with this.