I have been using cCleaner data wipe feature to erase contents on hdds, Now I need boot drive so I use Active @ KillDisk, it has 3 options:
Wipe unused clusters Erase areas of the hard drive that are not formatted and not currently used by the operating system (data has not been recently written there unless this is a recently deleted partition)
Wipe metadata and system files area Erase areas of the disk containing information about previous files on the volume and prevents recovery of files using past records of them
Wipe slack space in file clusters Erase slack space within files. Files are allocated a set amount of space by the OS, in certain increments (depending on the file system). Because files are usually never exactly the size of the space allocated to them, there may be unused space within a file that may contain traces of data. This algorithm wipes this space to remove these data traces.
Wipe unused cluster is marked by default, is this enough? cCleaner has option to set the wipe mode to "entire space".
In DiskKill if I mark all 3 options then it says "FULL Wipe, Slow" which is equivalent to cCleaners wipe mode "Entire Space" I guess.