CCC's Compare window was designed to highlight any substantive folder size differences between the source and destination. When you click the Compare button in CCC's toolbar, CCC will enumerate the current contents of the source and destination, then present a report of the size differences of each folder.
The comparison is not a byte-by-byte verification of files
The Comparison feature is not designed to perform an in-depth, byte-by-byte comparison of files on the source and destination, rather it's designed to be a simpler and more approachable analysis of size-based differences. We're specifically aiming to address the very common question, "Why is the size of the source and destination different?" If you would like to do a checksum-based verification of the files that were copied by your CCC backup task, click on the Source or Destination selector and choose the option to verify your files.
The comparison is not a preview of changes that CCC will make
Results in the Comparison window should not be used to determine what changes CCC will make to the destination. If you would like to see a preview of the changes CCC will make to the destination, click the Preview button in the toolbar instead.
The comparison shows some differences. What do these mean?
The comparison window shows the status of items on the source vs. the destination:
- This item is only present on the source
- This item is only present on the destination
- This item is different on the source and destination
- This item was modified since the task last ran
- This item is wholly or partially excluded or protected by a CCC task filter
- This folder could not be enumerated due to access restrictions
You can hover your mouse over the icons in the status menu to produce a tooltip that describes the status.
Common explanations for differences between the source and destination
If you're seeing unexpected differences between the source and destination, be sure to run your backup task to verify that CCC has recently attempted to update the destination.
Your startup disk is constantly being modified
If you're comparing your startup disk to its backup, you should always expect to see differences highlighted in the Compare window. This is not an indication that something is wrong, it's normal. macOS is constantly updating various cache and log files, and you'll see those differences even if you immediately compare the source and destination after running the backup task.
CCC doesn't copy virtual memory, Trash and other volume-specific system items
The disk usage on your startup disk does not reflect the amount of data that needs to be backed up; disk usage on the destination should be lower than disk usage on the source after making an initial backup of your startup disk. Special filesystem devices (e.g. filesystem snapshots) and some macOS service data either cannot or should not be copied to another volume. CCC automatically excludes these items to avoid compatibility problems and to avoid unnecessary disk usage. That list of exclusions is documented here: Some files and folders are automatically excluded from a backup task.
The largest and most notable excluded item is the /private/var/vm/sleepimage file. The sleepimage file contains the live state of your Mac's RAM, so it will be as large as the amount of RAM that you have installed. This file is potentially very large, changes constantly and it gets recreated on startup, so CCC excludes this file from every backup task.
CCC also excludes the contents of the Trash. If you prefer that CCC copy the contents of the Finder Trash, you can enable that in the Task Filter.
CCC's SafetyNet feature protects root-level items on the destination by default
By default, CCC's SafetyNet feature protects items that are unique to the root level of the destination. If the Comparison window shows files and folders that only exist at the root of the destination, you can disable the "Protect root-level items on the destination" setting to have CCC remove those items the next time you run the backup task.
Excluding content from the backup task does not cause it to be deleted from the destination
When you exclude an item from the CCC backup task, this tells CCC, "Do not copy that item". That does not, however, indicate that CCC should delete that item from the destination, e.g. if it had been copied there by a previous backup task. You can change this behavior by checking the box next to Remove excluded files in the sidebar of the Task Filter window.
The sum of folder sizes often does not agree with total disk usage
Disk usage is not a simple matter of adding the size of every file on a volume. Special filesystem devices (e.g. hard links) have always complicated this math, but more recently Apple has introduced more special filesystem devices that complicate this even further. The cloning feature in Apple's APFS filesystem can lead to a scenario where it appears that you have more data on the disk than it can possibly contain, and the filesystem snapshots feature can lead to scenarios where disk usage is higher than the total size of the files on that volume. APFS also supports "sparse" files, which consume less space on disk than their file size would suggest. CCC can preserve sparse files between APFS volumes, but HFS+ does not support sparse files, so these files consume more space on an HFS+ formatted backup disk. See these sections of CCC's documentation for additional details on working with these challenges:
- I heard that APFS has a "cloning" feature. Is that the same as what CCC is doing?
- Finder does not accurately represent the true disk usage of your files
- Understanding disk usage when using snapshots
- Snapshots and space concerns; Deleting snapshots
- Toggling snapshot support and setting a Snapshot Retention Policy
Two folders can have identical content yet different sizes
APFS cloning allows the user to instantly create copies of files on the same volume without consuming extra storage space. When duplicating a file in the Finder, for example, the file system doesn’t create copies of the data, rather it creates a second reference to the file that can be modified independently of the first file. The two files will share storage on the disk for portions of the files that remain identical, but changes to either file will be written to different parts of the disk. A common example of this occurs when people duplicate their Photos Library. The duplication occurs very quickly, and magically, the disk usage on the volume remains unchanged.
Those space savings cannot be propagated to another volume, e.g. to a backup volume. If you were to duplicate your Photos Library in your home directory, for example, and then back up that Pictures folder to your backup disk, the disk consumption of the Pictures folder on the backup disk would be twice that of the source, despite that the files in each folder are exactly identical.
Exporting the differences from a comparison report
Click the Export button in the Comparison window toolbar to export a tab-delimited report of the size differences. This report will include only the differences noted in the window.