You can use these parameters to create an exact copy of your whole partitions or hard disks, and not only the sectors that contain data. Note that the Back up unallocated space check box is available only when the Back up sector-by-sector check box is selected.
To make a sector-by-sector backup, check the Back up sector-by-sector parameter. By default the program copies only the hard disk sectors that contain data. However, sometimes it might be useful to make a full sector-by-sector backup. For example, you have deleted some files by mistake and want to make a disk image before trying to undelete them, because sometimes undeleting may create problems in the file system. Please note that this mode increases processing time and usually results in a larger image file because it copies used and unused hard disk sectors.
The Back up unallocated space option becomes available if you have selected the previous parameter, Back up sector-by-sector. By default, while performing sector-by-sector backup, unallocated space is not included into the backup file. Enabling this option will include all unallocated disk space into the backup.