See what is actually filling your disk
Storage settings break the disk down app by app, and explain that mysterious System Data entry.
Published: · Source: Genius Manu
When the disk fills up, guessing is a waste of time. Go to System Settings → General → Storage. The bar at the top splits the disk by category, and under it each app is listed with the space it occupies, largest first.
The entry that worries everyone is System Data. It is not a bug: it is caches, logs, temporary files, local snapshots and everything macOS cannot file under a neater heading. It grows and shrinks on its own, and the system trims it when space runs short. It is normal for it to take several gigabytes.
The number worth watching is simply Available.
The quickest win is emptying the Trash, which on a Mac holds deleted files until you say otherwise — so a “deleted” 10 GB video is still on the disk. Right-click the Trash in the Dock and choose Empty Trash. In the same Storage panel you can turn on Empty Trash automatically to have anything older than 30 days removed for you.
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Stop the desktop appearing every time you click the wallpaper
By default a click on any visible scrap of wallpaper sweeps all your windows aside. You can restrict it.
Set up the Dock the way you want it
Hide it when you are not using it, keep only the apps you open, and stop minimised windows piling up on the right.
Force quit an app that has stopped responding
The Mac equivalent of Ctrl+Alt+Delete for a single frozen app.