Some cloud storage service providers offer services that allow you to sync a local folder to "the cloud", and optionally choose to store those files only online, thus freeing up space on your hard drive. Some services that currently offer this functionality include:
- Dropbox Professional's "Smart Sync" feature
- Microsoft OneDrive's "Free up space" feature
- iCloud Drive's "Optimize Mac Storage" feature
- Google's "Drive File Stream" feature
Files that are only available online will typically have a "cloud" icon or badge in the Finder, e.g. iCloud: and Dropbox:
When you choose to have these services store your files only online, do so with the understanding that it's not possible to maintain a local backup of those files.
Online-only files can't be backed up
When you specify that a file stored by one of these storage services should reside only online, the local copy of your file is deleted from your Mac and replaced with a 0-byte placeholder file. If you attempt to open the placeholder file, the agent software for your storage service provider automatically downloads the data of the file to your Mac and the document opens. While this is a convenient feature that allows you to free up some space on your Mac, this feature removes files from your local storage, which means that CCC can't make a backup of these online-only files. Before using these online-only features, you should consider whether you are comfortable with not having a local backup of the files that you choose to store only in the cloud.
Placeholder files may be backed up, but may not function as placeholder files on the backup disk
As noted above, when you open a placeholder file in the Finder, the agent software downloads the original. Likewise, if you attempt to copy a placeholder file via the Finder from one volume to another, the agent software downloads the data to the source, then copies the original file (leaving the downloaded source file in place). CCC backups do not behave like Finder copies. And for good reason – if you have 1TB of online-only files on your 500GB SSD, you wouldn't want Dropbox or iCloud to download all of that data when CCC attempts to make a backup! Rather, CCC copies the placeholder files as they are, retaining all of the placeholder attributes of the source files. CCC makes a non-proprietary backup of your files; our goal is to make the destination files look exactly like the source files.
Some placeholder files won't function as placeholder files on the destination. OneDrive, for example, won't see a placeholder file that is outside of your OneDrive folder (i.e. on your startup disk) as a "true" placeholder file, and will not engage to download the file's data when you attempt to open it. CCC makes a best-effort attempt to not copy OneDrive placeholder files at all.
Dropbox's placeholder files function correctly when you back them up from an APFS volume to another APFS volume, and when you back them up from an HFS+ volume to another HFS+ volume. Because Dropbox uses a different proprietary technique for creating the placeholder file on each volume format, though, these placeholder files won't behave correctly when transferred from one filesystem format to another.
Google Drive File Stream uses yet another proprietary device for its placeholder files. These placeholder files can't be opened by any application other than Google Drive, so CCC does not attempt to back them up.