When Carbon Copy Cloner performs a block-level clone of your source volume to your destination volume, it proceeds in two stages: the cloning stage and the verification stage. Before it begins, both the source and destination are unmounted to prevent any external modifications to each filesystem. During the cloning stage, blocks are read from the source volume and written to the destination volume. While blocks are read from the source volume, a checksum of those blocks is calculated. In the verification stage, the blocks that were written to the destination are now read back and another checksum is calculated. The destination volume's checksum is then compared to the source volume's checksum. If these two values do not match, CCC will display an error message that "A block-level copy cannot proceed due to media errors".
Verification errors are occasionally the result of errors in hard drive drivers, defective hard drives, or defective cables, hard drive enclosures, etc. Resolving these hardware problems may allow your backup task to continue through to completion without error. Typically, though, verification errors are a result of the failure of one or more sectors of media on the source or destination volume.
What can I do to "fix" this problem?
The challenge in resolving this problem is determining what files are sitting on bad media. Because a block-level copy doesn't copy individual files, it doesn't know what file is associated with the block that it failed to read. Instead of repeating the backup task with the same settings, we recommend that you try the cloning task with a file-level clone instead. During the file-level backup, the exact files affected will be exposed and all the intact files will be backed up fine.