Your Cloud Folder Is Filling the C: Drive
A 2 TB cloud plan and a 256 GB laptop do not fit together. The setting that fixes it is one click, and the same click is how people end up with empty backups.
The symptom
Your local disk is full or nearly full, and the cloud folder is the largest thing on it.
Work down this list, not through it
Ordered by how often it turns out to be the cause. Run the check in the middle column before you change anything — each one takes under a minute, and it tells you whether that row is your problem or not.
| Cause | Confirm it’s this one | Fix |
|---|---|---|
| Files On-Demand is off Most common on OneDrive | Look at the icons beside your files. A blue cloud means online-only. A green circle with a white tick means it is pinned locally. | Turn on Files On-Demand, then right-click large folders and choose Free up space. Microsoft: "Online-only files don't take up space on your computer." Requires Windows 10 1709 or later, or Windows Server 2019. |
| Google Drive is set to Mirror rather than Stream Most common on Google Drive | Open Drive for desktop preferences and read which mode My Drive is using. | Switch to Stream. Files move to the cloud and are fetched when opened. Google: mirrored files "will always be stored on your computer and in the cloud"; streaming "uses very little space on your computer". |
| Folders pinned with Always keep on this device Common and easy to miss | Look for green filled circles rather than green outlines. A pinned parent folder pins everything inside it. | Right-click the folder and turn off Always keep on this device. Microsoft: files marked "Always keep on this device" have the green circle with the white check mark and "download to your device and take up space". |
| Everything has been opened at least once Very common over time | Files show as locally available even though you never pinned them. | Use Free up space on the parent folder to push them back to online-only. Microsoft: "When you open an online-only file, it downloads to your device and becomes a locally available file." |
| The space is not the cloud folder at all Worth ruling out first | Sort the whole drive by folder size. Compare the cloud folder against everything else. | Deal with whatever is actually largest - often a downloads folder or a virtual machine image. Not a vendor limit. Check before changing sync settings you may not need to change. |
This problem exists because of a mismatch that nobody designed on purpose.
Cloud storage plans grew enormously. Laptop disks did not — they got faster and, in a lot of cases, smaller. So the original arrangement, where your cloud folder was simply a folder on your computer that happened to be backed up, quietly became impossible.
A 2 TB plan cannot live on a 256 GB drive. Something has to give, and the mechanism that gives is the placeholder.
The fix is one setting
On OneDrive it is Files On-Demand. On Google Drive it is Stream rather than Mirror. Both do the same thing: your folder structure stays exactly as it looks, and the contents live in the cloud until you open something.
Microsoft's wording is unambiguous — online-only files do not take up space on your computer. Google's is equally direct: mirrored files are always stored on your computer and in the cloud, while streaming uses very little space.
Turn it on, then right-click the biggest folders and choose Free up space. Most people recover tens of gigabytes in under five minutes.
Learn the icons, because they are the whole story
OneDrive tells you the state of every file, and almost nobody knows how to read it.
- Blue cloud — online-only. Takes no space. Downloads when you open it.
- Green circle, white tick — you pinned this with Always keep on this device. It is downloaded and will stay downloaded.
- Opened but not pinned — downloaded because you used it, and eligible to be cleared again with Free up space.
That third state is where the disk quietly refills. You turn on Files On-Demand, reclaim 80 GB, and over six months open enough files that most of it comes back. Nothing is wrong; that is the design. Running Free up space on the parent folder occasionally is simply maintenance.
The trap is the pinned folder. Pinning a parent pins everything in it, including everything added later. One pinned folder high in the tree will undo all your work indefinitely, and it is invisible unless you go looking for it.
The same setting causes the opposite disaster
Here is the part that deserves more attention than it usually gets.
A streaming or on-demand folder is not a copy of your data. It looks exactly like one. Every folder is there, every file name is there, and the tree is complete.
Copy that folder to an external drive and you will have copied placeholders. The drive will report a few hundred megabytes for what appears to be a terabyte of work, and a great many people have not noticed that until the day they needed it.
If a local folder is your backup, on-demand is the wrong setting for it. Pick a subset that genuinely matters, mark it Always keep on this device, and let everything else stream. That is the arrangement that solves both problems at once: a disk that is not full, and a real local copy of the things you cannot afford to lose.
Check what is actually large first
One step before changing anything, because it costs a minute and occasionally saves the whole exercise.
Sort your entire drive by folder size and confirm the cloud folder really is the problem. It often is not. Downloads folders, virtual machine images, video project caches and old system restore points routinely dwarf it.
Changing your sync mode because something else filled your disk leaves you with a slower workflow and the original problem.
The default has changed
Worth knowing if you are helping someone else with this.
Microsoft states that OneDrive build 23.066 or later has Files On-Demand enabled by default, and that it requires Windows 10 1709 or later. So on a current machine this is increasingly not a setting someone chose — it is a setting they inherited, or one that was turned off years ago on an older machine and carried forward through every upgrade since.
Which means the question to ask is rarely "why did you configure it this way". It is usually "how old is this profile".
For the distinction between a synced folder and an actual backup — the thing that decides whether any of this protects you — see cloud storage vs backup.
If it is still broken
- Confirm you are looking at the real folder size on disk, which is what the placeholder mechanism changes, rather than the reported cloud total.
- Check for a second synced account or a SharePoint library syncing into the same profile.
- Look for a previous sync client left installed alongside the current one, keeping its own full copy.
- On Windows, check whether Storage Sense is already reclaiming space on a schedule and simply has not run.
- If the disk fills again within days, something is re-pinning content - look for a folder pinned at a level above the files you unpinned.
What we could not verify
- Whether Dropbox Smart Sync behaves identically to Files On-Demand in all cases - not established from primary documentation here.
- How long a locally available file remains cached before any automatic cleanup, on any of these clients.
- Whether files opened by background indexing or antivirus scanning are converted to locally available.
- Whether the 520-character Windows path limit interacts with placeholder files differently from full copies.
Open questions rather than criticisms. Where the vendor does not publish an answer we say so instead of filling the gap with a plausible guess, because a confident wrong answer is worse here than no answer.
Questions people ask
How do I stop OneDrive filling up my hard drive?
Turn on Files On-Demand, then right-click your largest folders and choose Free up space. Microsoft states online-only files do not take up space on your computer - they show as a blue cloud icon and download only when you open them. The change takes effect immediately.
What is the difference between Stream and Mirror in Google Drive?
Google documents that mirrored files are always stored both on your computer and in the cloud, using local hard drive space, while streaming keeps files primarily in the cloud and uses very little space. Mirror is the mode that fills a laptop; Stream is the mode that does not.
What do the OneDrive file icons mean?
A blue cloud means the file is online-only and takes no space. A green circle with a white check mark means you marked it Always keep on this device, so it is downloaded and stays downloaded. A file you have simply opened sits between the two - downloaded, but eligible to be cleared again.
Does Free up space delete my files?
No. It removes the local copy and leaves the file in the cloud, so it reappears when you open it. The important caveat is that you then need a connection to reach it - and if you were treating that local folder as your backup, you have just moved your only copy somewhere you cannot reach offline.
Why did my files disappear after I turned on Files On-Demand?
They have not been deleted - they are online-only, which looks like a normal file listing until you try to open one without a connection. This is also why a copy of a streaming folder is not a backup: you get a folder tree of placeholders rather than the files themselves.
Where this information comes from
Product details change often. Each item below links to the primary source we checked, so you can confirm anything that matters to your decision.
Vendors change limits and interfaces without announcing it. If a step here no longer matches what you see, tell us and we will re-check it.