Cloning a Time Machine backup

Can I use Carbon Copy Cloner to clone my Time Machine backup?

For important, albeit technical reasons, Carbon Copy Cloner will avoid copying the Time Machine "Backups.backupdb" folder in a file-level copy. It is often possible, however, to clone a Time Machine volume with CCC using a block-level copy:

  1. Enable the "Block copy" button in CCC's preferences: choose "Preferences..." from the Carbon Copy Cloner menu and check the box to "Indicate when a block-level clone is possible"
  2. Choose your source and destination volumes in CCC's main window
  3. Click the "Block copy" button to require a block-level copy
  4. Click the Clone button

Time Machine is a highly proprietary Apple device, and the block-copy tool that CCC uses to clone it is also Apple proprietary. Given the limited access that CCC has to the implementation details of both Time Machine and the block-copy tool, support for cloning Time Machine volumes is "Best Effort", and we cannot offer additional support for moving a Time Machine backup to a different volume when CCC's block copy method is unable to complete the task. If CCC refuses to permit a block copy or reports an error while attempting to clone a Time Machine volume, we recommend that you attempt to use Disk Utility's Restore functionality to clone the Time Machine volume instead.

Backing up Time Machine sparsebundle disk images (file-level backups)

When Time Machine is configured to back up a Macintosh to a network volume, Time Machine stores the backup in a sparsebundle disk image. CCC has no problem copying these sparsebundle disk image files. In fact, CCC quite capably copies only the bands within the sparsebundle that have changed, so you can add CCC to this type of setup for a second tier backup to an offsite network share.

CCC excludes the Backups.backupdb folder at the root level of a volume by default. However, you can choose "Choose a folder..." from CCC's Source menu and choose that folder directly to configure CCC to back up the sparsebundle disk images. Note that the only items in a Backups.backupdb folder that CCC will copy are sparsebundle disk images. Other folders, e.g. local Time Machine backups will be excluded. Further, CCC will only consider sparsebundle images for deletion in a Backups.backupdb folder on the destination. Other items in this folder on the destination will be protected from deletion.

In summary, when it comes to backing up a Time Machine backup, CCC will only back up sparsebundle disk images during a file-level copy in which you explicitly choose the Backups.backupdb folder as the source, whereas the entire Time Machine Backup will be copied during a block-level copy.

The technical reasons for excluding a Time Machine backup from a file-level copy

Time machine uses a feature of the HFS+ filesystem that was introduced in Leopard called "directory hard links". Like file hard links, a directory that is hard linked to another directory is not actually a new directory, it is simply a pointer to the previous directory. Time Machine uses these hard links to make references to entire directory trees whose contained files have not been modified. To properly clone a Time Machine backup, these directory hard links must be preserved.

Directory hard links are proprietary to Apple. Apple discourages their casual use by third party developers because, used incorrectly, they could create devastating looping directory structures that would render a volume useless. For this reason and others, support for directory hard links has not been implemented in CCC's file-level synchronization engine. CCC's block-level cloning engine, on the other hand, does preserve directory hard links, and is therefore capable of cloning a Time Machine volume.

Cloning a Time Machine volume that has failed sectors or filesystem corruption

If your Time Machine volume has become corrupted and Time Machine refuses to use it due to this corruption, you may turn to CCC thinking that cloning it to a new, "clean" volume will solve the problem. Unfortunately, CCC will not help you solve these kinds of problems. When you clone a volume with a block-level copy, filesystem corruption is preserved. In these cases, we concur with Apple's advice to start a new Time Machine backup from scratch.