Destructive Windows Script

[rbt]

Chris Lambacher wrote:
> The reason they are slow and tedious is that they need to write to
> every byte on the disk.  Depending on the size of the disk, there may
> be a lot of data that needs to be written, and if they are older
> computers, write speed may not be particularly fast.

OK, I accept that, but if you have a HDD that's 8GB total and it has 1GB 
of files, why must every byte be written to? Why not just overwrite the 
used portion?