Sometimes people see lots of available space on a big 8TB disk and can't resist using it for "overflow" items — large video files, archives of old stuff. "Hey, there's room for a CCC backup here too." CCC is designed to make non-proprietary, 1:1 copies. When you select a destination in CCC, CCC intends to make that destination identical to the source, and will do so by removing anything from the destination that isn't on the source.
SafetyNet is a safety mechanism that is designed to avoid immediate loss of data if you were to accidentally select a destination to a CCC backup task that is not intended to be managed exclusively by CCC. When SafetyNet is enabled, CCC will temporarily protect files and folders on the destination that aren't on the source by retaining these items in the SafetyNet.
With the introduction of the Backup Volume Setup Assistant in CCC 7 which asserts destination exclusivity, the SafetyNet feature is largely obsolete. SafetyNet remains applicable only in cases where the Backup Volume Setup Assistant cannot be presented (e.g. if it was disabled by the user, or when you select a folder as the destination). When you select a destination and the Backup Volume Setup Assistant is not presented, SafetyNet will be enabled by default if the selected destination volume or folder is not empty.
Best Practice: SafetyNet allows content to accumulate on the destination in a manner that is counter-productive to making backups. We recommend that you create a volume on your destination device for each source that you want to back up, and use that volume exclusively with CCC. When this exclusivity is defined, disable the SafetyNet feature – click on the Destination selector and choose "SafetyNet Off" from the SafetyNet submenu.
Versioning: SafetyNet is not designed to provide backup versioning, and is poorly-suited to that functionality. If you want to retain access to older versions of your files, enable snapshot support on your APFS-formatted backup volume, or enable snapshot support on your source volume if you're using a non-APFS destination.
When CCC encounters an item on the destination that should be replaced or removed (i.e. because the item is different, or not present on the source), that item will be temporarily retained within the SafetyNet.
When SafetyNet is off, any items on the destination that aren't on the source will be immediately deleted when your backup task runs (in other words, the destination will be treated like an ordinary backup). Bear in mind that this does not prevent you from accessing older versions of files in your backup when you have CCC Snapshot support enabled on your destination.
With this setting, CCC won't delete anything from the destination. If a file exists on the destination and not on the source, that file will be left in place on the destination. If CCC is updating a file on the destination, the older version of the file will be retained in the SafetyNet.
This setting is useful for archiving when your source has well-organized content. For example, if you store photos by project name, and you like to remove those projects from the source when the project is complete, you can use the Don't delete anything SafetyNet setting to retain those archived projects on the destination. We recommend making additional backups of archives – when the content is removed from the source and the only remaining copy is on the destination, that archived content lacks a hardware-redundant backup.
One cautionary note about using this setting: Because older files will accumulate on the destination, the archive size may exceed the capacity of the original source, making restore activity logistically difficult. Also, if your files are not well organized, you may find future restore activity to be quite tedious because everything you've deleted from the source will still be on the archive.
Snapshots enabled on an APFS destination
If you selected an APFS-formatted destination volume and CCC snapshot support is enabled on that volume, then CCC's SafetyNet feature is implemented via snapshots. At the beginning of the backup task, CCC creates a SafetyNet Snapshot on the destination. This snapshot captures the state of the destination volume before CCC makes any changes to it. When CCC proceeds to update the destination, it deletes and replaces files immediately as applicable. Because the files are retained by the SafetyNet Snapshot, those files are not permanently deleted until the snapshot is deleted. CCC deletes SafetyNet snapshots that are older than one week, or sooner if space is required for new backups.
SafetyNet snapshots are not part of backup versioning. Backup Snapshots store your backup history independently of SafetyNet: SafetyNet snapshots vs. Backup snapshots
Legacy SafetyNet folder
If you're backing up to a non-APFS volume, or if you have snapshot support disabled for an APFS destination, then CCC's SafetyNet is implemented as a "_CCC SafetyNet" folder on the destination. Items that would otherwise be deleted immediately are moved into time-stamped "archives" within this SafetyNet folder. These timestamps are purely for organizational purposes, do not interpret these as an indication that this feature supports versioning.
This section is not relevant when snapshot support is enabled on an APFS-formatted destination volume. See Snapshot thinning and pruning for more information on SafetyNet Snapshot retention.
If left unmanaged, the SafetyNet folder would eventually consume all free space on your destination volume. To prevent that from happening, CCC prunes the contents of the SafetyNet folder at the beginning of each task if free space is less than 25GB. This limit is automatically adjusted if a 25GB limit is too low for a particular source and destination.
SafetyNet pruning occurs at the beginning of a backup task, so CCC will never delete an item that was archived in the current backup task. Additionally, pruning is always limited to the contents of the _CCC SafetyNet folder that is at the root of the destination. CCC's pruner won't delete the current versions of files on your destination, nor anything outside of the scope of the CCC backup task. Lastly, archive pruning works at a macro level. If any portion of an archive exceeds a pruning threshold, the entire archive (e.g. the time-stamped folder) will be pruned.
You can customize the pruning settings by clicking the Advanced Settings button at the bottom of CCC's main window.
Prune archives in the SafetyNet when free space is less than [xx] GB
If your destination volume has less free space than the limit that you have specified, CCC will prune the oldest archives until the specified amount of free space has been achieved.
Auto Adjustment of the SafetyNet Free Space pruning limit
When the Auto adjust option is enabled (and it's enabled by default), CCC will automatically increase the free space pruning limit if your destination runs out of free space during the backup task. For example, if your pruning limit is set to the default of 25GB, and you have 25GB of free space at the beginning of the backup task, no pruning will be done at the beginning of the task. If that task proceeds to copy more than 25GB of data, however, the destination will become full. CCC will then increase the pruning limit by the amount of additional space that is required to complete the backup task. For example, if CCC needed to copy an additional 15GB of data when the destination became full, the pruning limit would be increased to 40GB.
Prune archives in the SafetyNet when they are older than [xx] days
CCC will prune archives that were created more than "xx" days ago.
Prune archives in the SafetyNet when they are larger than [xx] GB
Starting with the most recent archive, CCC will determine the amount of disk space that each archive consumes. When the cumulative total exceeds your specified limit, CCC will prune the remaining, older archives. If the newest archive is larger than the limit that you have specified, that archive will be completely removed.
Never prune archives in the SafetyNet
CCC will not automatically prune the contents of the "_CCC SafetyNet" folder at the root of the destination. SafetyNet content may eventually consume all of the free space on the destination, so you should periodically delete older archive folders to maintain enough free space for future backups. You may delete the contents of the SafetyNet folder without harm to the rest of your backup – SafetyNet is not part of the backup strategy.