Note: Selective sync is an older Dropbox feature. Selectively synced files and folders still use space on your hard drive, while non-synced items are only available on dropbox.com.
Smart Sync is a newer, more advanced version of this feature. With Smart Sync, you can set files and folders as "online-only." Online-only files use practically no space on your hard drive, and you can still see them in the Dropbox folder on your computer.
To turn on selective sync:
Unchecking a shared folder will stop that folder from syncing to your computer. However, you’ll still be a member of that shared folder, and it will still appear in your account on dropbox.com. The shared folder will also remain active for other members of that folder. To remove yourself from a shared folder, you can either leave the shared folder from dropbox.com or, if you're the owner of a shared folder, you can unshare it.
If you’re on a Dropbox Business team, your admin may also have turned on team selective sync. This feature allows team admins to create a default selective sync setting for a team folder (or a folder in the team space). This default then applies for all new members of a Dropbox Business team with access to that folder.
If you have a Dropbox Business or Professional account, you can also use Smart Sync to manage hard drive space on your computer. Smart Sync allows you to see all of the files in your Dropbox on your computer, but only downloads the ones that you want. You can use both Smart Sync and selective sync together, or choose just one.
If you use selective sync and the feature stops working as intended, learn how to resolve common issues.
If you're seeing a general issue with Dropbox sync, try our master troubleshooting article.
Usually, selectively syncing files will free up space on your hard drive within minutes (as long as your computer is online and able to sync to Dropbox). However, there is a known limitation with macOS 10.13 (High Sierra):
Apple's operating system macOS 10.13 (High Sierra) uses a new type of file system called APFS. With APFS, the operating system takes snapshots of the file system and available hard drive space. These snapshots may not update after you've selectively synced Dropbox files. This means that hard drive space you freed up with selective sync may not be immediately reflected or available if this snapshot hasn't updated.
This hard drive space should eventually be freed up by the OS, but the amount of time this will take can vary. This isn't a behavior specific to Dropbox, but instead the designed behavior of macOS.