Carbon Copy Cloner maintains a list of certain files and folders that are automatically excluded from a backup task. The contents of this list were determined based on Apple recommendations and years of experience. The following is a list of the items that are excluded along with an explanation of why they are excluded.
Items prefixed with a "/" indicate that they will only be ignored if located at the root of the volume.
Items postfixed with a "/*" indicate that only the contents of those folders are ignored, the folders themselves will be copied.
Items postfixed with a "*" indicate that the filename will be matched up to the asterisk.
Filesystem implementation details
- .HFS+ Private Directory Data*
- Network Trash Folder
These items only show up if you're running an older OS than what was used to format the source volume, and on some third-party implementations of AFP and SMB network filesystems. These items should never, ever be manipulated by third-party programs.
These items record volume-specific preferences, e.g. for Spotlight, Time Machine, and a custom icon for the volume. Feedback on the exclusion of these items is welcome. Because they are volume-specific preferences, the exclusion of these items from a day-to-day backup seems most appropriate.
Apple-proprietary data stores
These items are Apple-proprietary data stores that get regenerated when absent. Attempting to copy these data stores without unmounting the source and destination is not only futile, it will likely corrupt them (and their respective apps will reject them and recreate them).
The DocumentRevisions data store is used by the Versions feature in macOS. The Versions database stored in this folder contains references to the inode of each file that is under version control. File inodes are volume-specific, so this dataset will have no relevance on a cloned volume.
Volume-specific cache files
Copying these caches to a new volume will render that volume unbootable. The caches must be regenerated on the new volume as the on-disk location of system files and applications will have changed. macOS automatically regenerates the contents of these folders when CCC is finished updating the backup volume.
NetBoot local data store
In the unlikely event that your Macintosh is booted from a Network device, macOS will store local modifications to the filesystem in this folder. These local modifications are not stored in a restorable format, therefore should not be backed up. In general, you should not attempt to back up a NetBooted Mac.
These items represent special types of folders on macOS. These should not be backed up, they are dynamically created every time you start the machine.
Quota real-time data files
When these files are copied to a destination volume using an atomic file copying procedure, the macOS kernel will prevent the destination from being gracefully unmounted. The contents of these files is never accurate for the destination volume, so given the kernel's unruly behavior with copies of these files, CCC excludes them. According to the quotacheck man page, these files should be regenerated every time a quota-enabled volume is mounted (e.g. on startup). We have not found that to be consistently true. If you're using quotas, run
sudo quotacheck / after restarting from your backup volume or a restored replacement disk to regenerate these files.
Large datastores that are (or should be) erased on startup
- /macOS Install Data
macOS stores virtual memory files and your hibernation image (i.e. the contents of RAM are written to disk prior to sleeping) and temporary items in these folders. Depending on how you use macOS and your hardware configuration, this could be more than 50GB of data, and all of it changes from one hour to the next. Having this data for a full-disk restore does you absolutely no good — it makes the backup and restore processes take longer and the files get deleted the next time you boot macOS.
Moving an item to the trash is typically considered to be an indication that you are no longer interested in retaining that item. If you don't want CCC to exclude the contents of the Trash, you can modify each task's filter:
- Choose Copy Some Files from the popup menu underneath the Source selector
- Click the Inspector button adjacent to that same popup menu to reveal the Task Filter window
- Uncheck the box next to Don't copy the Finder's Trash
- Click the Done button
Time Machine backups
These folders store Time Machine backups. Time Machine uses proprietary filesystem devices that Apple explicitly discourages third-party developers from using. Additionally, Apple does not support using a cloned Time Machine volume and recommends instead that you start a new Time Machine backup on the new disk.
Corrupted iCloud Local Storage
iCloud leverages folders in your home directory for local, offline storage. When corruption occurs within these local data stores, macOS moves/renames the corrupted items into the folders indicated below. macOS doesn't report these corrupted items to you, nor does it attempt to remove them. CCC can't copy the corrupted items, because they're corrupted. To avoid the errors that would occur when trying to copy these corrupted items, CCC excludes the following items from every backup task:
- Library/Mobile Documents.*
Files included in this section are application-specific files that have demonstrated unique behavior. The kacta and kactd files, for example, are created by antivirus software and placed into a special type of sandbox that makes them unreadable by any application other than the antivirus software.
The "com.apple.loginwindow" item can be found in each user home folder. Excluding this item prevents the applications that were open during the backup task from opening when you boot from the backup volume. This seems appropriate considering that Apple intends the feature to be used to open the applications that were in use when you log out, restart or shutdown, not at an arbitrary point during the backup task.
- /Library/Application Support/Comodo/AntiVirus/Quarantine
CCC SafetyNet folders
When CCC's SafetyNet feature is enabled, CCC creates a _CCC SafetyNet folder at the root of the selected destination volume or folder. When CCC encounters an item on the destination that does not exist on the source, or an item that will be replaced with an updated item from the source, that item gets placed into the SafetyNet folder rather than being deleted immediately. The SafetyNet folder is literally a safety net for files on your destination. If you accidentally delete a file from the source and you don't realize it until after your backup task runs, you'll find the item in the SafetyNet folder. Likewise, if you accidentally specify the wrong volume as a destination to a CCC backup task, the mistake does not catastrophically delete every file from the selected destination; you simply recover the items from the _CCC SafetyNet folder.
The protection that the SafetyNet folder imparts is specific to the volume upon which the SafetyNet folder resides. As such, CCC never includes the contents of the _CCC SafetyNet folder in a backup task. So, for example, if your hard drive fails and you restore your backup to a replacement disk, the _CCC SafetyNet folder is automatically excluded from that restore task. If you have several tasks backing up to separate folders on a backup volume, for example, the _CCC SafetyNet folders that are created in those subfolders would not be included in a secondary backup task that copies your backup disk to a third disk.