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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 backing up a Time Machine backup set in a file-level copy. It is 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

Do you really need to clone your Time Machine volume? After all, it is a backup...

If the only thing that your backup drive is used for is Time Machine, you could just start "fresh" on the new volume. We recommend that you keep the old drive around for a few weeks in case you want to retrieve a file modified in the backup window of your previous Time Machine backup, but a fresh start is a full backup.

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.