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When your Mac’s startup drive (called Macintosh HD by default, even if it’s an SSD) runs out of room, there are other places to store files. Even if you can’t upgrade internally, you can plug in external drives. Or you can keep more files in the cloud, which will probably mean upgrading your iCloud storage plan to more than the basic 5GB. Go to System Preferences > Apple ID > iCloud, click Manage at the bottom right, then Buy More Storage (or Change Storage Plan). 200GB is a good option for many people, at $2.99 per month.