You have the possibility to set advanced settings for the deletion of the free space under Settings/SafeErase free space
SafeErase cluster tips
Clusters are the smallest hard disk units, where data can be saved. An entire unit will be occupied, even if the file is smaller than the smallest cluster. For example, if the smallest unit is 64 Kbytes in size and the file to save only 5 Kbytes small, the remainder of the hard disk unit will stay unused and there can be no access to it. Yet, file fragments of previously deleted files might still be located in this "free" area.
SafeErase MFT entries
For NTFS-file systems you can SafeErase the MFT entries (Master File Table). Every file is represented in the MFT by an entry. While using MFT entries, it is possible for data rescue programs to find and restore previously deleted data. To avoid this security loophole, O&O SafeErase offers the possibility to delete these entries. This way, unauthorized persons cannot reconstruct your data even with special software. In FAT file systems this setting deletes all FAT entries.
SafeErase free space
Deleted files are not really gone forever. When files are deleted, the area on the hard disk where the deleted data is recorded is simply marked by the operating system as free and open for the placement of other data. If this doesn't happen, data can be found and read. Therefore, it is important to SafeErase this free space.
To apply the settings, confirm the dialog with OK.