Backing up to a disk image

This documentation is for an older version of CCC. You can find the latest version here.
Last updated on 19 juli 2023

We discourage the use of writable disk image destinations

Writable sparse disk images are particularly sensitive to connectivity loss between the disk image volume and the disk image file. Reports of disk image corruption have grown steadily worse, especially since the introduction of APFS, and especially when the disk image is hosted on NAS storage. If you're currently using a disk image as part of your backup strategy and it's working for you, you're welcome to continue using it. This functionality is still present within CCC, and we will continue to support it in scenarios where the disk image is working reliably. As you make changes to your backup strategy in the future, however, and especially if you encounter trouble mounting a disk image or accessing its content, we recommend that you migrate away from writable disk images and back up directly to a folder or volume on the underlying storage.

Related documentation

A disk image is a single file that contains the entire contents of another hard drive (except for the free space). When you want to access the contents of that filesystem, you double-click on the disk image to mount the disk image as if it were an external drive attached to the machine.

To back up to a new disk image:

  1. Choose your source volume from the Source selector
  2. Choose New disk image... from the Destination selector
  3. Provide a name and choose a location to save your disk image
  4. If you plan to back up to this disk image again in the future, set the image format to one of the read/write formats. If you want a read-only disk image for archival purposes, set the image format to one of the read-only formats.

To back up to an existing disk image, select Choose disk image... from the Destination selector and locate your disk image, or simply drag and drop the disk image file onto CCC's Destination selector box.

Read/write "sparseimage" disk images

Use of this older disk image format is not recommended, we only make it available as a potential workaround for some SMB NAS devices

A sparseimage disk image is a type of read/write disk image that grows as you copy files to it. In general, sparse disk images only consume as much space as the files they contain consume on disk, making this an ideal format for storing backups. Please note that sparseimage files are monolithic and potentially very large files. If the underlying filesystem has a 2TB file size limit and the sparseimage file reaches that limit, the sparseimage file cannot be grown. In most of these cases the sparseimage file becomes corrupted when the underlying filesystem limit is reached, so we don't recommend this disk image format for large data sets.

Read/write "sparsebundle" disk images

A sparse bundle disk image is similar to a sparseimage insofar as it grows as you add data to it, but it retains its data in many smaller files inside of a bundle rather than inside a single file.

Running out of space on a sparse disk image

CCC reported that the destination is full, but the underlying disk has plenty of free space. CCC initially sets the capacity of your disk image to the amount of free space on the underlying disk. If you have freed up some space on that disk since you created the disk image, you can manually expand the capacity of the destination disk image in Disk Utility. Choose Resize... from the Images menu in Disk Utility, select your destination disk image, then expand it as desired. We recommend that you do not expand the disk image such that it is larger than the capacity of the underlying disk.

The disk image file is larger than the amount of data it contains, why? Sparseimage and sparsebundle disk images grow as you add data to them. They do not, however, automatically shrink when files are deleted from them. As a result, the amount of disk space that the disk image file consumes will not necessarily reflect the amount of data that they consume. To reclaim disk space that is occupied by the free space on your sparse disk image, CCC will compact the disk image before attempting to mount it if the free space on the underlying volume is less than 25GB, or is less than 15% of the total disk capacity. In most cases, you do not need to compact the disk image yourself, but this functionality is documented here so you'll understand why you might see CCC spending time "Compacting the destination disk image" at the beginning of a backup task.

If you would like to compact a disk image manually, drop the disk image file onto this application†: Compact Sparse disk images. Be sure to unmount the disk image volume if it is already mounted. Also, note that the compacting process can take a while (e.g. an hour for a 100GB disk image on a locally-attached volume). Finally, be sure that your system is running on AC power. The system utility that compacts the disk image will refuse to run while the system (e.g. a laptop) is running on battery power.

† Big Sur (and later) users: Right-click on the application and choose "Open" to get past the GateKeeper restriction. Or if you prefer, you can use the command-line hdiutil utility to compact the disk image (e.g. hdiutil compact "/path/to/disk image.sparsebundle").

CCC applies more aggressive SafetyNet pruning to disk image volumes

When you configure a task to back up to a new disk image, CCC will configure the task's SafetyNet pruning to prune anything older than 1 day. You are welcome to change these settings, but we have found that more aggressive SafetyNet pruning will avoid excessive use of disk space on the underlying device, and will reduce the need to compact the disk image.

Please keep in mind that SafetyNet is not intended to offer access to older versions of your files, it is a safety mechanism that is designed to avoid the loss of data on an errantly-selected destination volume. SafetyNet is generally not applicable to disk image backups because the disk image is typically dedicated to the backup task. However, enabling SafetyNet with even a very aggressive pruning limit does offer a modicum of protection in cases where you've accidentally removed files from the source.

If you're looking for a solution that retains older versions of your files and your source volume is APFS-formatted, consider CCC's snapshot functionality instead. Snapshots are disabled on disk image destinations by default, but you can enable snapshot support either on the disk image volume or on the source volume.

Read-only disk images

Read-only disk images cannot be modified without invalidating the built-in checksum, therefore they are a good container for storing archived material. Compression rates vary on the content of your source, but you can typically expect to reduce the size of your disk image by about half when using compression. There is a subtle behavior that you should take note of when considering this option as a space-saving measure: CCC will first create a read/write disk image, copy the selected items to it, then convert the disk image to read-only compressed. In this case, you will actually need twice the space on your destination as the items to be copied consume on the source.

Encrypting disk images

If any of the data that you are backing up is sensitive, and if your backup device may be in an insecure location, encrypted disk images can improve the security of your backup. CCC offers 128 bit and 256 bit AES encryption to encrypt disk images. To create an encrypted disk image, select one of the encryption levels from the Encryption menu. After you click on the OK button, you will be prompted to specify a passphrase for the new disk image, and CCC will give you an opportunity to save the passphrase in your own keychain. CCC will also store the passphrase in a private keychain so the disk image can be mounted automatically during scheduled backup tasks.

Note: If you create a read-only, encrypted disk image, the intermediate disk image that CCC creates is NOT encrypted. This intermediate disk image file is deleted once the final, read-only, encrypted disk image has been created, but it is not shredded. Take this into consideration when choosing your destination media. If the destination may be placed in an insecure location, use Disk Utility to securely erase free space on the underlying destination volume after you have created your encrypted disk image archive.

Running a backup task whose destination is a disk image on the startup disk

If you specify a disk image that resides on your startup disk as the destination to a scheduled task, CCC will impose some more conservative requirements on this task. To proceed with this configuration, one of the following requirements must be met:

  • The amount of free space on the startup disk is at least 1GB larger than the amount of consumed space on the source volume.
  • The disk image won't grow, e.g. it is a .dmg file, not a sparseimage or sparsebundle disk image.

These requirements avoid a scenario in which the startup disk runs out of free space, causing instability on macOS. If you cannot accommodate the free space requirement, we recommend that you create a .dmg disk image in Disk Utility (choose File > New... > Blank Disk image, set the image format to read/write disk image). Disk Utility will pre-allocate exactly as much space as you request, and CCC will gladly use this disk image without fear of filling up the startup disk.

Snapshots and Disk Images

When creating a new disk image, CCC will format the disk image to match the source volume. For better performance on APFS-formatted disk images, CCC will disable snapshot support on the destination disk image volume if:

  • The backup task was originally configured to create a new disk image
  • Snapshots are currently enabled for the destination disk image
  • The snapshot retention policy limit for SafetyNet snapshots is set to the default value of 7 days

When CCC disables snapshots on that destination disk image volume, it explicitly sets the SafetyNet limit in the snapshot retention policy to 0. If you subsequently re-enable snapshot support on that volume without changing the SafetyNet limit back to the default, then snapshots should remain enabled (because the three logical conditions are no longer matched).

If you would like to enable snapshot support on your disk image and keep it enabled, be sure to either leave the SafetyNet limit set to 0, or change it to anything other than 7. If you ever change the SafetyNet retention value for that disk image back to 7 (or reset the values to defaults), CCC will again disable snapshots on the disk image when the task next runs.

A message for new Mac users coming from the Windows world

Backups on a Windows system are very different from those on a Macintosh. If you're coming from a Windows background, the term "imaging" and the concept of making a disk image backup is probably familiar to you. Restoring from disk image backups is made simpler on Windows because the startup environment is built around them. That's not the case for a Macintosh. When you create a disk image backup of your Mac's startup disk, the logistics of restoring that backup are actually fairly complicated. Due to these complications, we don't recommend using a disk image as your primary backup on a Mac. Disk images are useful for storing a backup of your user data on a network volume, but for your Mac's startup disk, we recommend that you back up directly to a disk that is attached to your Mac; not to a disk image.

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