Block-level copies vs. file-level copies
Cloning an entire hard drive to another drive 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 hard drive 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 significantly faster than file-level copying, and is therefore the preferred method for disk to disk clones.
To benefit from a block-level copy, the following criteria must be met:
You must choose to "Delete items from the target that don't exist on the source" during the clone.
You must be able to unmount both devices (there cannot be any open files on either drive and you cannot be booted from either drive). Do not actually unmount the drives prior to the clone.
The target drive must be at least as large as the source drive.
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 (assuming you didn't merge two operating systems), 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). When possible, CCC will always prefer a block-level clone. However, don't lose sleep 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.
