You have 2TB of cloud storage and a laptop with a 256GB drive. Both of these things can be true at the same time, and the feature that makes it work goes by a different name on every service: selective sync, smart sync, files on-demand, stream files, optimize storage.
They fall into two distinct categories that behave very differently, and knowing which you're using explains most of the odd things cloud folders do.
The two approaches
Selective sync lets you choose which folders download to this computer. The folders you don't choose are invisible here — they're not in your file listing at all. They exist on the server and on other devices.
Smart sync / files on-demand shows you everything, but downloads nothing until you open it. Every file appears in your folder listing with its correct name and size, but most of them are placeholders occupying almost no space. Open one and it downloads in a moment.
The difference in one line: selective sync hides things, smart sync shows everything as ghosts.
Smart sync is the better default for almost everyone, because "I can't find the file" is a worse problem than "the file took two seconds to open."
How placeholders actually work
A placeholder is a small entry that carries a file's name, size, type, and icon, but not its contents. Your operating system treats it as a file. When an application tries to read it, the sync client intercepts the request, downloads the contents, and hands them over.
This is why the file size shown for your cloud folder can be enormous while the disk space used is small. Both numbers are correct; they're measuring different things.
Two consequences that catch people out:
Opening an online-only file requires internet. Obvious in principle, forgotten constantly in practice — usually on a plane.
Some operations trigger mass downloads. Searching file contents, running a virus scan, indexing, or a backup program pointed at your cloud folder can each cause thousands of files to download at once. This fills your disk and consumes bandwidth without any obvious cause. If your disk suddenly filled and you don't know why, this is the usual explanation.
The icons, decoded
Broadly consistent across services:
Cloud outline — online-only. Not on this computer. Needs internet to open.
Green tick or solid circle — downloaded and available offline. Takes disk space.
Circular arrows — currently syncing. Don't shut down mid-sync.
Pinned or filled marker — explicitly marked to always keep on this device.
Red or grey X, or an exclamation mark — a problem. Something failed to sync, and this is the icon worth paying attention to. Click it.
There's a third state worth understanding: a file that downloaded because you opened it, but isn't pinned. It's local now, and the system may remove it later to reclaim space. That's fine for a document you read once and bad for something you're relying on having offline — which is why pinning exists.
Pinning: the setting that matters
Marking files or folders as "always keep on this device," "available offline," or "pinned" tells the client to download them and never evict them.
Pin these:
Anything you're actively working on. Working from online-only files means every open and save waits on the network.
Anything you need without internet. Travel documents, reference material, offline work.
Files your applications expect to be there. Some software — design tools with linked assets, databases, accounting packages, development environments — behaves badly when files appear and disappear. Pin the folder or move those files out of cloud sync entirely.
Leave online-only:
- Archives and finished projects
- Large media libraries
- Anything shared with you that you only occasionally reference
Pin at the folder level rather than file by file. It's less to manage, and folder-level pinning survives new files being added.
Per-device configuration
Configure each device separately, according to what it's for:
Desktop with a large drive: pin generously, or mirror everything. A full local copy is useful — it's not a backup, but it's a second copy that would survive an account problem.
Laptop: stream everything, pin current projects. This is the classic case for smart sync.
Work machine: pin only what you need. Streaming keeps the local footprint small and reduces what's exposed if the machine is lost.
Phone or tablet: almost nothing local, except specifically-marked offline files for travel.
The mistake is copying one machine's configuration to another. A laptop pinned like a desktop fills up within months.
When you'd still choose selective sync
Smart sync is better in general, but selective sync has real uses:
Hiding folders you never want on this device. Work folders on a personal machine, someone else's shared library, a huge archive. Not seeing them at all is cleaner than seeing thousands of ghost entries.
Reducing clutter on a shared computer. If several people use the machine, hiding folders is simpler than explaining placeholders.
Working around software that mishandles placeholders. Occasionally an application copes badly with on-demand files. Excluding that folder from sync entirely, and managing it manually, is a reasonable workaround.
Very old operating systems that don't support the placeholder mechanism at all.
The critical safety note: deselecting a folder in selective sync removes it from your computer but not from the cloud. This is by design and it's fine. But if you ever have the reverse instinct — deleting local files to save space on a synced folder — that does delete them everywhere. The two operations look similar and have opposite consequences.
Freeing up space without deleting anything
Most clients offer a "free up space" option, either per-folder or automatically for files you haven't opened recently. It converts local files back to placeholders. The cloud copy is untouched.
This is the correct way to reclaim space on a synced folder. Do not drag files out of the folder or delete them to make room — that removes them everywhere.
Automatic space reclamation, where offered, is worth enabling on laptops. It quietly evicts files you haven't touched in a while, and re-downloads them if you need them again.
Troubleshooting the common problems
A file won't download. Check the connection, check you have free disk space, check the file isn't larger than your remaining space. Restart the sync client; this fixes a surprising proportion of issues.
Files keep re-downloading. Something is reading them. Antivirus, backup software, a search indexer, or a media application scanning for content. Exclude the cloud folder from those tools' scans.
The disk filled up unexpectedly. Same cause. Find the tool that's touching everything and exclude the folder, then free up space.
Sync is stuck on one file. Usually a permissions problem, a file that's open in another application, an illegal character in the name, or a path that's too long. The client's error detail usually names the file. Rename it, close whatever has it open, or move it out and back.
Files vanished from a device. Check whether selective sync was changed, or whether the folder was un-shared by its owner. Check the cloud web interface — if it's there, it's a local sync issue, not a data loss.
The sensible default configuration
For most people, on a laptop:
- Turn on the on-demand mode — smart sync, files on-demand, or stream files.
- Pin your current projects folder and anything you need offline.
- Leave archives, media, and old work online-only.
- Exclude the cloud folder from antivirus and backup software scans.
- Enable automatic space reclamation if offered.
That gives you the full library visible and searchable, a small local footprint, and current work always available.
Feature names, behaviours, and availability vary by service and change with updates. Check your provider's current documentation for specifics.
Next in this series: How to Share Cloud Files Safely: Links, Permissions, and Expiry