“Your startup disk is almost full” — what do I do?
The short answer
Your startup disk is the Mac's internal drive, and it's nearly out of room. That matters more than it sounds: macOS needs free space to work. Apps slow down, downloads fail, and system updates refuse to install until there's room. Freeing 20 to 30 GB usually makes all of it go away.
Free space right now, in this order
Empty the Trash and Recently Deleted in Photos. Neither frees anything until emptied.
Open System Settings → General → Storage and look at the Recommendations: Store in iCloud, Optimize Storage, Empty Trash Automatically.
Delete installers you've already used. Check ~/Downloads for .dmg and .pkg files.
Restart. It clears memory swap and temporary files, often several gigabytes.
Look inside System Data. It's usually the biggest single block. What's in it →
Why a full Mac misbehaves
macOS writes memory to disk when RAM runs short, and apps write caches as they work. With no room, both fail quietly. That's why a full Mac feels slow long before it runs out completely. macOS updates also need a lot of free space to install, often 20 GB or more.