Initially, snapshots do not inherently consume space. When you create a snapshot, the disk usage on the volume containing the snapshot remains unchanged. However, because the snapshot retains references to every file on the volume, space is not freed when you delete a file. Suppose you have a 100GB hard drive with 80GB of content. You create a snapshot, then move 20GB of files to the Trash and empty the Trash. The resulting disk usage is still 80GB. That 20GB of space is not freed until the snapshot is deleted.
This free space behavior is an important factor to consider when you decide whether to enable snapshots for any particular volume, including your startup disk. If you have a hard drive that is particularly full, then maintaining snapshots on that volume may not be a practical solution. In contrast to Time Machine, CCC offers a lot of flexibility in whether snapshots are enabled for a particular volume, and how those snapshots are maintained over time. Additionally, CCC allows you to find and delete specific snapshots with ease.
To delete a snapshot in CCC:
- Select Volumes in the sidebar
- Select a volume (to remove a snapshot from the startup disk, select the volume named "Data" or "Macintosh HD - Data")
- Select one or more snapshots in the Snapshots table
- Press the Delete key
Total snapshot disk usage is not a simple sum of individual snapshot sizes
The disk usage indicator in the center of the window will show the space consumed by all snapshots. Separately and unrelated, the "size" value listed for an individual snapshot is an indication of the amount of space that will be freed if you delete only that snapshot. "Size" is not a great title for that column, but "Amount of space that would be freed if you were to delete this individual snapshot" is too long. Also, note that the size of the snapshot does not indicate the total amount of data referenced by the snapshot.
You may be tempted to add up all of the individual snapshot sizes and compare that to the total snapshot disk usage presented in the chart in the center of the window. Don't do that, these values are not directly comparable. Calculating the "size" of an individual snapshot is complex and dynamic – as you delete snapshots, the "size" (again, "space that would be freed if you were to delete just that individual snapshot") of other snapshots may change as those snapshots become the last reference holder for files on the disk.
Consider this over-simplified example. Suppose you started with an empty disk on Monday, and created a 2GB file. Your backups then run on Monday, Tuesday and Wednesday. Your backup disk now has the following:
- 2GB file visible in Finder
- Snapshot (Monday): 0KB
- Snapshot (Tuesday): 0KB
- Snapshot (Wednesday): 0KB
- Total snapshot disk usage: 0KB
The 2GB file is still present on disk, so despite that each snapshot has a reference to that 2GB file, deleting any individual snapshot would not free any space because the file hasn't been deleted.
Next, you delete the file on Thursday and run your backup again. Your backup disk now has the following:
- Snapshot (Monday): 0KB
- Snapshot (Tuesday): 0KB
- Snapshot (Wednesday): 0KB
- Snapshot (Thursday): 0KB
- Total snapshot disk usage: 2GB
Each snapshot shows "0KB" because deleting any *individual* snapshot would not free any space, and that's because the Monday/Tuesday/Wednesday snapshots each has a reference to the 2GB file. If you delete all three snapshots, that would remove the last reference holders to the 2GB file, and that would free the space. Likewise, if you delete both the Monday and Tuesday snapshots, no space would be freed, but now the Wednesday snapshot would show a size of 2GB. That's because the Wednesday snapshot is now the last reference holder to that 2GB file. The Thursday snapshot doesn't have references to any files because it was created after the 2GB file was deleted, so it's size is always 0KB.
This video demonstrates the concept:
▶️ Learn more about snapshots and disk usage concerns
If you want to see how much space would be freed if you were to delete more than one, but not all of the snapshots, select a swath of snapshots and then right-click on the selection. CCC will calculate the disk usage of that collection of snapshots, which will (typically) be larger then the sum of their individual sizes.