Carbon Copy Cloner logs its activity to a file located at /Library/Logs/CCC.log. You can use this log to verify the productivity of your scheduled tasks. To view CCC's log file, choose "CCC Log" from the Window menu. Beta versions of CCC will produce more output than non-beta versions. Additionally, you can manually enable debug logging by choosing "Enable Debug Output" from the Help menu.
Debugging output is only useful to the developer, so don't enable debug logging unless instructed to do so for support purposes. CCC imposes a size limit of 512KB on the log file. When this limit is reached, the log is rolled over.
This value indicates the data written that was actually consumed as disk space. For example, suppose you have a plain text file that contains nothing more than "I really love cheese". That file that occupies a single 4Kb block (4,096 bytes) on the disk, but it only consumes 20 bytes of the block. The "Data copied" figure represents the accumulation of those 20 byte values. This figure will *not* correspond to the value of disk usage indicated in Disk Utility (not even close in most cases), Disk Utility reports the number of blocks occupied multiplied by the size of the block.
Note that the " Total data in file set" figure is also not necessarily representative of all the disk usage on your source volume. This figure does not include unmodified extended attributes — to determine the size of extended attributes, CCC would have to read each one. Reading each extended attribute regardless of whether it was modified would consume unnecessary CPU cycles and would make the backup task take much longer than necessary. This is especially notable when making a backup of an OS X system volume, because 3-5GB of disk space is consumed by compressed data stored in extended attributes.