Why iCloud Fills Up Your Mac Disk: How Optimize Storage Really Works
Room in the Cloud, None on the Disk: Where the Confusion Starts
Short answer: your iCloud storage plan (5 GB, 200 GB…) and your Mac's disk space are entirely separate. Every file in iCloud Drive can also have a local copy on your Mac; the "Optimize Mac Storage" setting removes those local copies only when the disk starts filling up and only in the order macOS chooses — not when you ask.
That is why "I have a 2 TB iCloud plan but my Mac is full" is so common. Worse, the instinctive fix — turning iCloud Drive off — fills the disk completely, because macOS downloads everything stored in the cloud. This guide explains how the two kinds of storage differ, what the optimize setting really does, and how to undo the mistake that fills the disk.
Key Takeaways
- Your iCloud plan is space in the cloud; your Mac's SSD is separate space. Freeing one does not free the other.
- "Optimize Mac Storage" does not remove local copies immediately; it does so only under disk pressure, in an order macOS picks.
- Turning iCloud Drive OFF fills the disk: macOS downloads everything from the cloud. Check your free space before you do it.
- The local iCloud Drive folder lives at `~/Library/Mobile Documents` and is hidden in Finder by default.
- Deleting a file from iCloud Drive deletes it from ALL your devices; to free space locally, use "Remove Download" instead.
- If Desktop and Documents folder syncing is on, everything in those folders counts both in the cloud and locally.
Two Separate Storages: Where the Confusion Begins
An iCloud storage plan is space you buy on Apple's servers: 5 GB free, then 50 GB, 200 GB or 2 TB. Your Mac's disk is the SSD inside the machine. They are independent, and cleaning one does not directly affect the other.
The confusion comes from this: a file you put in iCloud Drive sits both in the cloud and on your Mac by default. Moving a 100 GB archive to iCloud does not take those files off the Mac — it just creates a second copy. Your cloud plan does not overflow, but your disk does not empty either.
The "Documents" or "iCloud Drive" line you see in System Settings > General > Storage is exactly those local copies. To see your cloud total you have to look at System Settings > Apple Account > iCloud; the two screens measure two different things.
What "Optimize Mac Storage" Actually Does
With the setting on, macOS is allowed to remove local copies of iCloud Drive files when it needs to. The file still appears in Finder with a small cloud icon next to it, and double-clicking downloads it again and opens it. That is why you do not lose anything.
But there is an important detail: macOS does not do this immediately. The setting is a permission, not a command. The system evicts local copies only once the disk genuinely starts filling, and in an order it chooses itself, starting with files you have not opened in the longest time. That is why "I turned on optimization and nothing happened" is such a common observation.
If you want space now, act by hand: right-click a large folder in Finder and choose "Remove Download". That deletes its local copy immediately while the files stay safe in the cloud. It is ideal for video archives, old project folders and scan files.
iCloud Photos: Where Do the Originals Live?
With iCloud Photos on, your whole library uploads to the cloud. In the Photos app under Settings > iCloud you choose between two options: "Download Originals to this Mac" or "Optimize Mac Storage".
The first keeps every full-resolution photo and video on the Mac. That makes sense if this is your main machine and you back it up, but a 500 GB library means 500 GB of disk. The second leaves originals in the cloud and keeps downsized versions locally that are good enough for the screen; when you edit or export a photo, the original downloads at that moment.
One thing to note: even with optimization on, the Photos library does not shrink to zero. Thumbnails, previews and local copies of recent shots accumulate. A 200 GB library can still take 20-40 GB in its optimized state. That is normal, not a fault.
The Classic Mistake That Fills the Disk
The most common mistake is this: the disk fills, you assume "it must be iCloud", and you turn iCloud Drive off. That does the opposite. When iCloud Drive is disabled, macOS offers to download every file that lives in the cloud but not locally; accept and your already-full disk fills completely, decline and you lose access to those files from the Mac.
The right order is: turn optimization on, then remove local copies of large folders by hand, and only then delete what is genuinely unnecessary from iCloud. Be careful with that last step — a file deleted from iCloud Drive is deleted from your iPhone, iPad and other Macs too. To free space locally only, use "Remove Download" rather than delete.
The local iCloud folder sits under `~/Library/Mobile Documents` and is hidden in Finder by default, which is why it shows up in unexpected places in folder-size tools. Disk Mop's Disk Analysis module walks the disk folder by folder including hidden ones, surfacing exactly this kind of buried accumulation, while the Large Files module shows which individual local copies are the most expensive.
Frequently Asked Questions
If I delete a file from iCloud, does it disappear from my Mac?
Yes. iCloud Drive is a synced folder: a file you delete on one device is deleted everywhere and stays in iCloud's Recently Deleted for 30 days. To remove only the local copy, right-click and choose "Remove Download".
I turned on optimization but the disk is still full. Why?
Because the setting is a permission, not an instant command. macOS evicts local copies only when the disk comes under pressure. For an immediate result, remove downloads of large folders manually in Finder.
Is "System Data" in the storage screen related to iCloud?
Partly. That line covers many things, including iCloud caches and sync temporary files. But Time Machine local snapshots and system logs count there too, so it cannot be explained by iCloud alone.
Should I turn off Desktop and Documents syncing?
Not necessarily. Having it on is good for backup. Be careful when turning it off though: macOS leaves the contents inside iCloud Drive and your local Desktop can look empty. Always read the on-screen explanation before you confirm.
Verdict
iCloud and your Mac disk are two separate accounts: freeing cloud space does not relieve the disk, and cleaning the disk does not empty your plan. For the disk, the right order is to enable optimization, remove downloads of large folders by hand, and never turn iCloud Drive off "to fix it".
To see what local accumulation is left, Disk Mop's Disk Analysis and Large Files modules sort the whole disk by size including hidden folders, and the Disk Treemap view shows at a glance which folder is really taking the space.
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