Backing up to a disk image

This article is for an older version of CCC. You can find the latest version here.
Product: 
ccc3

Sometimes backing up your data to a single external hard drive is just too messy and difficult to organize. Suppose, for example, that you have three computers in your household, and everyone backs up to the same external hard drive. Where do you put each person's backup? The single hard drive can't maintain multiple operating systems, OS X does not support that. You could split the drive into three partitions, but that isn't very flexible at all. You could clone each person's drive to a folder on the drive, but that's a little difficult to restore from.

A good solution in a scenario such as this is to use disk images as the medium of your backup. 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. Carbon Copy Cloner 3 leverages disk images to provide you the flexibility of storing several complete, bootable* backups on a single shared external hard drive. (* Disk images themselves are not bootable, but you can mount them and restore their content to a physical hard drive to produce a bootable, exact replica of the original).

To back up to a new disk image:

  1. Choose your source volume from the Source menu
  2. Choose "New disk image..." from the Destination menu.
  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 menu and locate your disk image.

Read/write "sparse" disk images

A sparse 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.

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. A sparse bundle may be desirable if the underlying filesystem upon which you save the disk image has a file size limitation (such as FAT32).

Sparseimage and sparsebundle disk images will automatically grow, but they will not automatically shrink

Sparseimages 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 sparsebundle disk image, 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).

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. Whether to choose read-only versus read-only compressed comes down to time and space. If your destination is tight on space and you have a bit of extra time, you can compress the disk image. 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. When using a block-level copy, this limitation does not apply, CCC will create a read-only compressed disk image in one step.

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 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.

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

These requirements avoid a scenario in which the startup disk runs out of free space, causing instability on OS X.

"CCC refused to mount the sparsebundle disk image because doing so would put its contents at risk of data loss"

CCC will refuse to 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. Here's a little background to understand why:

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, a new kind of disk image introduced in Mac OS 10.5.

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. CCC extends and improves this protection by refusing to mount a sparsebundle disk image if that disk image resides on a filesystem that does not support the F_FULLFSYNC file control, regardless of the current OS that you are running.

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. When you encounter this error, copy the sparsebundle disk image to another network sharepoint, or ask CCC to create a new sparse disk image file (sparse disk images are not the same as sparsebundle disk images).

"CCC is unable to proceed with the backup task because the underlying destination volume was mounted by another user"

When a scheduled task running on OS X Lion is backing up to a sparsebundle disk image hosted on a network volume, this message will appear if the network volume was already mounted by a user other than the System Administrator. To avoid this problem, unmount the network volume, then run the backup task again. Note that CCC will automatically mount the network volume when the task is scheduled to run, so it is not necessary to keep the network volume mounted or to mount the network volume prior to the backup task's scheduled run time.