Optimised Storage and Store in iCloud: what they actually do
Both leave something behind that looks like your file. What that placeholder can and cannot do, and the two capabilities it quietly gives up.
Three separate settings share almost the same name and do different things. Reading the storage bar first tells you whether any of them is worth switching on. The one that matters is Optimize Mac Storage in iCloud Drive, which lets macOS strip the contents out of files it thinks you are not using. The file stays where it was. The data goes to iCloud. Two capabilities go with it.
The naming is genuinely bad, so it is worth sorting out before anything else.
Three settings, two of them called Optimize
| Setting | Where it lives | What it does |
|---|---|---|
| Store in iCloud | Storage recommendations | Moves Desktop and Documents into iCloud Drive, plus Photos and Messages |
| Optimize Storage | Storage recommendations | Removes watched Apple TV purchases and older mail attachments |
| Optimize Mac Storage | iCloud Drive settings | Removes the contents of less used iCloud Drive files when space is short |
The third is the one people mean and the one that causes the questions. The first two are narrow and reversible. The third changes how every file in iCloud Drive behaves.
What eviction actually does to a file
The word for it is eviction. It is more precise than offloading or removing.
An evicted file is still there. The name, the location, the icon, the modification date, all present. What has gone is the data inside it. macOS leaves a stub pointing at the full copy in iCloud Drive. The file behaves normally right up until something tries to read its contents, at which point it downloads.
Nothing is deleted. Eviction and deletion look similar in Finder and are not the same operation. The file remains in iCloud Drive at full size throughout and opening it locally restores it. This is the part that works exactly as advertised.
macOS evicts in reverse order of how recently you used things. The files that go first are the ones you have not touched in longest.
What an evicted file can no longer do
Here is what the setting does not tell you. It is the reason to think carefully before switching it on.
Still works
While online
- Appears in Finder in its usual place
- Opens by downloading on demand
- Keeps its name, date and folder
- Syncs to your other devices
Stops working
While evicted
- Time Machine cannot back it up, because the data is not local
- Spotlight cannot search inside it, for the same reason
- Nothing opens it with no connection
- Neither loss is announced anywhere
The backup consequence is the serious one. A file that has been evicted is not in your Time Machine backup, because there is no local data for Time Machine to copy. Turn the setting on and files begin quietly leaving your backup as they are evicted. No notification appears and nothing in Finder marks them.
The Spotlight consequence is smaller and more annoying. Searching for a phrase inside a document only works if the document is local. An evicted file will not turn up in a search that should have found it.
Why it does not keep your Mac empty
The most common complaint about this setting is that it seems not to work. People switch it on, expect iCloud Drive to stop taking up space, then watch the Mac download everything anyway. Space that has been freed this way is counted as available rather than free, which muddies the reading further.
That is correct behaviour. Optimize Mac Storage acts only when space runs short. Given room, macOS keeps the full contents of iCloud Drive locally, because having files ready is better than fetching them.
It is a relief valve rather than a policy. If you want iCloud Drive to hold things your Mac does not, this setting will not deliver it. No setting in macOS will. That is a reasonable thing to want and it is not what a syncing service is for.
How to keep a file downloaded anyway
The counterweight is per file and per folder. It is worth knowing before anything else here.
Right click anything in Finder and choose Keep Downloaded. That pins the item locally however short space becomes. The decision moves back to you, rather than resting on a heuristic about how recently you opened something.
Worth pinning: current project folders, anything you work on while travelling and anything you rely on Time Machine to protect. The last one matters most, since pinning is what keeps a file in your backup.
Why none of this is a backup
iCloud Drive is a syncing service. That sounds like a technicality and it is the whole difference.
Read more: What IMAP and POP actually store on your Mac and why neither is a backup.
Sync propagates. Backup preserves. Those are opposite jobs and only one of them protects you from your own mistakes. Delete a file on your iPhone and it leaves iCloud Drive and your Mac within seconds, correctly and immediately, because propagating the change is precisely what you asked for.
So the combination worth avoiding is Optimize Mac Storage switched on with Time Machine as your only backup. Evicted files sit outside the backup. Synced files are one accident from gone on every device at once. The gap between those two facts is where people lose things.
If the disk is genuinely too small, the honest answers are an external drive or a larger Mac. Eviction manages the symptom and quietly costs you a backup while it does it.
Common questions
Why is iCloud Drive still filling my Mac with Optimize turned on?
Because the setting only acts when space runs short. With room available macOS keeps the full contents locally, which is what it is meant to do. It is a relief valve rather than a policy of keeping your Mac empty.
Does turning it on delete anything?
No. Eviction removes the data from a file while leaving the file itself visible in place. Nothing is deleted from iCloud Drive. The file returns to full size when you next open it.
What happens to an evicted file when I have no internet?
You cannot open it. The placeholder is there and the contents are not, with nothing local to fall back on. That is the single most important thing to know before enabling it on a laptop.
Can I stop specific files being evicted?
Yes. Right click a file or folder in Finder and choose Keep Downloaded. That pins it locally regardless of how short space becomes, which is worth doing for anything you work on away from a connection.
Is iCloud Drive a backup?
No. It is worth being blunt about that. It is a syncing service. Delete a file on one device and it goes from all of them, which is the opposite of what a backup does.
Should I use it at all?
On a desktop Mac with a decent connection and a small disk, it is reasonable. On a laptop that travels or on any Mac where Time Machine is your only backup, the trade offs are worse than the space is worth.
Sources
Every date, version number and path above is taken from these pages rather than from memory.
- iCloud Drive in Sonoma: Optimise Mac Storage or not The Eclectic Light Company
- Optimize storage space on your Mac Apple Support
- Why is iCloud auto downloading files, community thread on when eviction happens Apple Support Communities