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

If enabled in CCC's preferences, CCC will indicate if the source and destination volume selections qualify for a block-level copy. During a block-level copy, CCC unmounts the source and destination volumes, reformats the destination volume to match the source, then CCC proceeds to copy blocks of data from the source to the destination. Once all of the blocks from the source have been transferred, CCC then reads all of the blocks that were written to the destination and calculates a checksum. This checksum is compared against a checksum of the source volume. If the checksums match, the destination volume is then expanded to reach the extent of the partition (so if you cloned a 1TB drive to a 2TB drive, the destination volume would be 2TB when you're finished).

While each method produces functionally the same result, an initial backup will take less time with a block-level copy than a file-level copy. Because a block-level copy transfers the blocks of data in order, in perfect sequence, the heads of the hard drive are not tasked with the intensive seeking operations that are required for a file-level copy.

Requirements for a block-level copy

To take advantage of a block-level copy for a volume-to-volume clone, the following criteria must be met:

  1. The source and destination must both be actual volumes (i.e. not a subfolder on a volume).
  2. The source and destination must both be locally-attached, HFS+ formatted volumes.
  3. CCC 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.
  4. The destination volume must be at least as large as the source volume*, plus two blocks (usually 8KB). In some cases you can use Disk Utility to shrink the source volume to meet this requirement.

When these criteria are met, CCC will activate the "Block copy" button adjacent to the Destination popup menu. You can click on this button to explicitly require a block-level copy. If these criteria are not met, the "Block copy" button will remain disabled, and a tooltip indicating why the criteria are not met will be displayed when you hover your mouse over the "Block copy" button.

* In some cases, cloning to a smaller volume is actually possible. If you click on the padlock button and authenticate, then reselect the destination volume, CCC can perform a more in-depth analysis of how data is arranged on the source and determine whether the smaller destination volume can accommodate a block copy of the source.

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 destination 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 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 OS X from a disk image):

  1. The source volume cannot be the startup disk that you are currently booted from.
  2. The source volume must be an actual volume (i.e. not a subfolder on a volume).
  3. The source volume must be a locally-attached, HFS+ formatted volume.
  4. CCC must be able to unmount the source volume. Do not actually unmount the source volume prior to the clone.
  5. The underlying destination volume must have enough capacity to accommodate the amount of space used on the source volume.

Cloning volumes larger than 2TB

CCC uses the OS X system service "Apple Software Restore" to perform block-level copies. On versions of OS X prior to Lion, Apple Software Restore may be unable to complete a block-level copy of a volume larger than 2TB. This failure can occur without reporting an error, and despite a successful verification. If you are running an OS older than Lion, CCC will not permit a block-level copy of a volume that is larger than 2TB.

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. If the destination volume is invalid, CCC will reformat the destination. Due to the nature of a block-level copy, the destination volume's filesystem directory will not match the actual files on its filesystem unless the entire block-level copy procedure completes.

If you abort a block-level clone during the Verification phase, the destination volume may appear intact and function just fine, but will not show the true capacity of the destination 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.

Frequently Asked Questions about the block-level copy

Is a block-level copy better than a file-level copy?

A block-level copy is faster for an initial backup. If your destination volume already has a lot of the source volume's content (i.e. from a previous backup), a file-level update will actually be faster than a block-level copy. As far as the end result is concerned, the two methods are equal. Both methods can produce a bootable backup of OS X.

Does the block-copy method copy unallocated blocks? Are deleted files copied?

Some unallocated blocks will be transferred, but it is not a goal of CCC to copy all unallocated blocks. CCC will copy up to the "high tide" mark on the source volume, that is, to the last block that contains actual file data. If the source volume's free space is fragmented, then some unallocated blocks will definitely get transferred. Any unallocated blocks past the high-tide mark, though, will not be transferred. If you're hoping to recover deleted files from your source volume, we recommend that you attempt this from the original source volume, not the cloned volume.

Can the block-level copy be used for a forensics-level backup of a volume?

No, the block-level copy is not an appropriate tool for a forensics copy of the disk. Most importantly, CCC doesn't make any effort to prevent the mounting of the source volume, nor does it impose read-only requirements when the source is mounted, so you cannot be sure that changes were not made to the source volume after evidence collection.

If there are filesystem problems or corrupted files on the source, will these be propagated to the destination?

Not only will these problems be propagated to the destination, it is also unlikely that a block-level copy will complete successfully if there are corrupted files (i.e. bad sectors) on the source volume. If you suspect problems with your source volume, use CCC's file-level copy. If/when errors are encountered, CCC will note the affected files and move on, copying the files that are intact. The Cloning Coach will then appear at the end of the task to offer advice on addressing the problems with your source.