Optimize Mac Storage in iCloud: what it removes and when
· 6 min read
Optimize Mac Storage removes the local copies of files that already exist in iCloud, keeping the ones you have used recently. It only ever touches iCloud Drive and, as a separate setting, your Photos library. It does nothing to apps, caches, System Data or anything outside those two places, and it does not act at all until the disk is genuinely short of space.
That last point is why most people turn it on, watch the free space number not move, and conclude it is broken. It is not broken. It is waiting.
Where the switches actually are
There are two of them, in different places, and they do different jobs.
iCloud Drive. Open System Settings, click your name at the top, then iCloud, then iCloud Drive. The Optimize Mac Storage option lives with iCloud Drive’s own settings. Apple has moved it a level in and out across recent macOS versions, so if it is not on the first screen, open iCloud Drive itself and look there.
Photos. This one is not in System Settings at all. Open the Photos app, press Command-comma for its settings, and choose the iCloud tab. You pick between Download Originals to this Mac and Optimize Mac Storage. Turning on the iCloud Drive option does not change this, and vice versa.
There is also a Store in iCloud button under System Settings, General, Storage, in the Recommendations block. That button is a shortcut that switches on Desktop and Documents syncing along with the optimize options. It is convenient and it is also the reason people end up with their Desktop syncing to iCloud without quite having decided to.
What it removes, and what it leaves completely alone
When the disk gets tight, macOS starts evicting local copies of files that are safely stored in iCloud, oldest and least-used first. The file stays visible in Finder with a cloud icon beside it; opening it downloads it again.
What it will consider:
- Files in iCloud Drive, including your Desktop and Documents folders if you have those syncing
- Photos and videos in your Photos library, if the Photos setting is set to optimize, replaced locally by smaller versions
What it will never consider:
- Applications
- Anything in System Data: caches, logs, swap, temporary files
- Your Downloads folder, which is not synced
- Any folder outside iCloud Drive, including project folders, virtual machines and media libraries you keep elsewhere
- Files belonging to another user account on the Mac
This is the honest answer to “why is my Mac still full”. If the space is in Xcode data, container images, old iPhone backups or a Photos library you have deliberately set to keep originals, this feature will never reach it. What System Data is and how to shrink it covers the part it cannot help with.
When it actually runs
Optimization is reactive, not scheduled. macOS evicts files when free space falls low enough that it needs to, and not before. On a Mac with a couple of hundred gigabytes free, you can leave the setting on for a year and never see a single file evicted.
That is reasonable behavior: keeping files local is better when there is room for them. It just means you cannot use this feature to free space on demand. If you need space now, you have to remove something yourself.
You can evict individual files by hand, which is the closest thing to on-demand control. Right-click a file or folder in iCloud Drive in Finder and choose Remove Download. The item stays in iCloud and in the Finder listing, and the local copy goes. Double-clicking it later brings it back.
The catch that matters: backups
An evicted file is not on your disk. Time Machine backs up what is on the disk. So files that iCloud has optimized away are not in your Time Machine backup, and the same is true of any other backup tool that works from local files.
This is not a bug and it is not hidden, but it does quietly change what your backup means. If your Documents folder is in iCloud Drive and half of it has been evicted, your local backup covers the half that happens to be downloaded. iCloud is your only copy of the rest, and iCloud is sync, not backup: delete a file on one device and it goes everywhere.
If you want a real second copy, keep originals downloaded on one machine, or archive the important material somewhere that does not depend on eviction state.
Turning it off, and the trap in doing so
Switching Optimize Mac Storage off tells macOS to bring everything back down. That means the download is as large as the total you have in iCloud Drive or Photos, it needs the free space to land in, and on a slow connection it can take days. Check both numbers before you flip the switch.
Turning off Desktop and Documents syncing is the sharper edge. When you disable it, macOS leaves your files in iCloud Drive and gives you fresh empty Desktop and Documents folders locally. Nothing is lost, but it looks exactly like everything has been lost, and people then delete things from iCloud in a panic. Download the folders from iCloud Drive first, confirm they are on the disk, and only then turn the setting off.
The related recommendations, briefly
The same Recommendations block in Storage settings offers two other things worth understanding.
Optimize Storage is a narrower feature than its name suggests: it removes Apple TV movies and shows you have already watched, which you can download again from your purchases. It has nothing to do with your own files.
Empty Trash Automatically deletes anything that has been in the Trash for thirty days. That is longer than most regret, so it has no real downside.
Also expect the free space figure to lag. Evicted files often show up as purgeable rather than free for a while, which is why the bar in Storage settings and the number in Finder can disagree. Purgeable space explains what that means and when it resolves.
Seeing what is really taking the space
Before deciding iCloud is the answer, it is worth knowing what the disk actually contains. System Settings, General, Storage breaks it into categories with an information button for each one, which is a reasonable starting point.
For a fuller reading, Crumb audits the whole Mac read-only, including the parts Storage settings summarizes as System Data: caches, logs, swap, temporary files and leftovers, plus Time Machine local snapshots and old iOS backups. The audit and one full cleanup are free. It is a disk cleaner, not a sync tool, so it says nothing about what lives in iCloud; it tells you whether iCloud is the right thing to be adjusting.
Questions
Does Optimize Mac Storage delete my files? It removes the local copy of files that are already in iCloud, leaving them in place in iCloud and visible in Finder. It never removes something that exists only on your Mac. The risk is not deletion, it is availability: an evicted file needs a network connection to open.
Why has nothing been freed since I turned it on? Because macOS only evicts when free space is genuinely low. Until then it keeps everything local, which is the better default. If you need space today, this setting will not provide it.
What happens to evicted files when I have no internet? They cannot be opened. Finder shows the cloud icon and the download fails. Before traveling, open the files you will need so they are downloaded, or turn optimization off on that machine while you have bandwidth.
Does this help with a Photos library that is taking up a lot of room? It can, and it is the intended tool for it, but only if the Photos setting specifically is set to optimize. There are other reasons a Photos library grows, including duplicates and its own caches. The Photos library taking up space goes through them.