iCloud Drive files are on your Mac until you evict them

The most common misunderstanding about iCloud is that turning it on frees space. It does not. It makes a copy in the cloud and leaves the local copy exactly where it was, and macOS removes local copies only when it decides the disk is under pressure.

Where the local copies live

du -sh ~/Library/Mobile\ Documents/* 2>/dev/null | sort -h | tail -10

~/Library/Mobile Documents is iCloud Drive on disk, one folder per app that uses it. Desktop and Documents, if you have that option turned on, are inside it too, which is why the home folder can look small while the disk is full.

What Optimise Mac Storage really does

It gives macOS permission to remove local copies of files you have not opened recently, keeping a placeholder that downloads on demand. Two things about that are worth knowing:

• It is reactive, not proactive. Nothing is evicted while there is plenty of free space. Turning the setting on today may free nothing today.
• You cannot choose what goes. macOS decides by recency. If you want a specific folder gone from the disk now, the setting will not do it.

Forcing a file or folder back to the cloud

In Finder, right click a file or folder in iCloud Drive and choose Remove Download. The file stays visible with a cloud icon, the local copy goes, and the space is free immediately. This is the control most people are looking for and it is one menu item down.

find ~/Library/Mobile\ Documents -type f -size +100M 2>/dev/null \
  | head -20

That lists the largest local copies, which are the ones worth evicting first. A downloaded file and an evicted one look identical in Finder apart from the cloud icon, so the sizes are the only reliable guide.

The trap with Desktop and Documents

Turning the Desktop and Documents option off does not delete anything, but it does move files: they go back to a local folder, and until that finishes, both copies exist. On a disk that is nearly full this can make things worse before it makes them better, so it is worth doing with some free space in hand rather than as a last resort.

Whether iCloud is even the right answer to a full disk is a separate question with a cost attached, and iCloud storage against an external drive prices all the options including doing nothing. What the storage recommendations do covers the setting itself.