Disk images are not bootable backups. To create a bootable backup, you must back up to a hard drive that is attached directly to your Mac. We recommend that you only use a disk image if you are backing up to a network volume, and we recommend using locally-attached storage for your primary backups.
A disk image is a single file residing on your hard drive 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. We recommend using disk images sparingly. If you're backing up to a network volume and your Mac and the NAS device are connected to the network via ethernet, then a disk image may be a good fit. In most cases, however, disk images are not a great choice for your backup strategy.
To back up to a new disk image:
- Choose your source volume from the Source selector
- Choose New disk image... from the Destination selector
- Provide a name and choose a location to save your disk image
- 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.
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. Use of this older disk image format is only recommended when backing up to non-AFP network volumes on an OS older than macOS Sierra. 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.
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. We recommend this disk image format for most scenarios.
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.
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.
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.
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.
If your Mac is running an OS older than macOS Sierra, CCC will refuse to save or mount a sparse bundle disk image if the underlying filesystem that the disk image file resides upon does not support the F_FULLFSYNC file control. Most filesystems support this file control, but the SMB file sharing protocol does not. Most people that encounter issues with creating a sparsebundle disk image on a network volume are encountering issues because the network volume is mounted via SMB.
Starting in Mavericks, Apple's preferred file sharing service is SMB. As a result, if you attempt to connect to a network volume, Finder will use SMB to establish that connection unless you explicitly specify AFP as the protocol to use. In this configuration, a sparse bundle disk image will not work, and CCC will issue an error. To avoid this error, connect to the network volume explicitly using AFP:
- Eject the network volume if it is currently mounted
- Choose Connect to server from the Finder's Go menu
- Type in "afp://yourserver.local" (changing the hostname, of course), then click the Connect button and mount the network volume
- Go back to CCC and choose Choose disk image... from the Destination selector, then select the sparsebundle disk image on your network volume
Why can't I use a sparsebundle disk image on a filesystem that does not support the F_FULLFSYNC file control?
When your computer writes a file out to the hard drive, the data usually goes to a "write buffer" — a small portion of RAM that is installed on the circuit board of the hard drive. By accumulating smaller write operations onto this RAM chip, the hard drive can increase overall write performance by writing large blocks of cached data to the physical media all at once. While this write buffer improves performance, it also carries a risk. If the power fails or the disk's connection to the computer is suddenly broken between the time that data was written to the buffer and when the buffer is flushed to the disk, your filesystem will have an inconsistency. Filesystem journaling typically mitigates this risk, however it doesn't offer enough protection for Apple's sparsebundle disk image type.
In Mac OS 10.5, Apple implemented the F_FULLFSYNC file control for network servers and clients. The F_FULLFSYNC file control is a command that is sent to the hard drive after some (or all) write operations that tells the disk to immediately flush its cache to permanent storage. To provide better protection for data on sparsebundle disk images, Apple disabled support on Mac OS 10.6 for using sparsebundle disk images that reside on filesystems that do not support the F_FULLFSYNC file control. Apple relaxed this requirement in macOS 10.12 (Sierra).
You are likely to encounter this error condition if your sparse bundle disk image is hosted on a pre-Mac OS 10.5 Macintosh or various Network Attached Storage (NAS) devices (especially SMB). When you encounter this error, copy the sparsebundle disk image to another network volume, or ask CCC to create a new sparseimage disk image file (sparseimage disk images are not the same as sparsebundle 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 other reset the values to defaults), CCC will again disable snapshots on the disk image when the task next runs.
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.