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Backing up everything: The venerable Full Volume Clone

 

Carbon Copy Cloner's most basic and revered task is to create a "carbon copy" bootable clone of a hard drive. CCC 3 takes this to a new level with Apple Software Restore, a low-level tool developed by Apple and built-in to Mac OS X. Apple Software Restore is the most highly regarded tool for performing disk to disk clones, and Carbon Copy Cloner brings all the functionality and more to your desktop. To clone a hard drive using Carbon Copy Cloner, use the "Backup everything" method listed under Cloning options:

Screenshot of Backup Everything method

The "Backup everything" method will attempt a block-level clone of your source volume if you have indicated to "Delete items from the target that don't exist on the source". If you have not selected that option, or if a block-level clone cannot proceed (see below), this cloning method will perform a file-level copy of everything on the source volume to the target volume. If the target volume already contains some of the content of the source volume (e.g. from a previous backup), that content will be efficiently updated. The end result in both cases is an exact, bootable backup of your source volume.

Options to the "Backup everything" method

This cloning method has one option: Whether to delete items from the target that don't exist on the source prior to cloning. If there is a file that exists only on the target volume and not on the source, CCC will delete that item when this option is selected. This option is important to consider when your goal is to create a bootable clone. To learn more about the effect of this option on bootability, read the Why must you use the "Delete items on the target that are not on the source" cloning option to make that volume bootable? section of the documentation.

Block-level copies vs. file-level copies

Cloning an entire volume to another volume can be achieved in two ways: 1) Copying every file individually from one volume to the other or 2) Copying the underlying blocks from one volume to the other. These two methods are called "file-level copying" and "block-level copying", respectively. While each method produces functionally the same result, block-level copying is almost always faster than file-level copying.

To benefit from a block-level copy for a volume-to-volume clone, the following criteria must be met:

  1. You must choose to "Delete items from the target that don't exist on the source" during the clone.
  2. You must be able to unmount both volumes (there cannot be any open files on either volume and you cannot be booted from either volume). Do not actually unmount the volumes prior to the clone.
  3. The target volume must be at least as large as the source volume.

If any of these criteria are not met, CCC will automatically fall back to a file-level clone. A file-level clone will yield the same result -- a bootable clone, but it will take a little longer.

While a block-level clone is rarely required to insure the fidelity of your backup, some people find comfort in the fact that, block-for-block, the resulting target is precisely like the original (this is simply impossible with a file-level clone). Don't lose sleep, though, if meeting the requirements for a block-level clone is untenable for your particular backup or cloning situation. The file-level backup engine in CCC has been tuned and honed to bring the best performance and fidelity possible among Mac OS X backup utilities.

To benefit from a block-level copy for a volume-to-disk image clone, the following criteria must be met (and note that you cannot boot Mac OS X from a disk image):

  1. You must choose the "Backup everything" cloning method.
  2. You must be able to unmount the source volume and the source volume must not be the startup disk. Do not actually unmount the source volume prior to the clone.
  3. The underlying target volume must have enough capacity to accommodate the amount of space used on the source volume.

Aborting a block-level clone

If you click on the "Stop" button while CCC is in the middle of a block-level clone, CCC will immediately abort the copying procedure and remount the source volume. The target volume, however, will not be remounted. Due to the nature of a block-level copy, the target volume's filesystem directory will not match the actual files on its filesystem unless the entire block-level copy procedure completes. You will have to reformat the target volume using Disk Utility before it can be remounted.

If you abort a block-level clone during the Verification phase, the target volume may appear intact and function just fine, but will not show the true capacity of the target volume. For more technical details on the phases of a block-level copy, see this discussion in the Public Discussion section of the Bombich Software Help Desk.

Do I need to clone, or just backup?

Often people clone when all they really need is a backup. When you clone a volume, every file is copied, and this can take a fair amount of time. If you just recently backed up your entire drive, use the "Incremental backup of selected items" method to backup your data instead. That method copies only the items that have changed, thus your backup is much faster. If you're moving to a new hard drive, cloning your machine prior to repair, etc. then the cloning method is the right choice for you.

Exceptions to the "Backup Everything" rule

In block-copy mode, CCC omits absolutely nothing from the backup task. In fact, it isn't even possible to omit files from a block-level copy. Some people use this to their advantage to backup Time Machine backup volumes. In file-level copying mode, though, the "Backup Everything" cloning method does exclude a very small subset of devices and folders that ought not be backed up in file-level mode for a number of reasons:

The typical reasons that these files should not be backed up in file-level copy mode are that they involve "implementation details" of the HFS+ filesystem, or that they will have no relevancy when copied in this mode (e.g. Spotlight). The Time Machine Backups.backupdb exclusion is covered in more detail in the Cloning a Time Machine backup section of the documentation. Despite these exclusions, rest assured that CCC is backing up everything required to preserve your data and your Mac OS X installation.