HD space question

[Jezae]

I've recently had to replace a hard-drive and it got me thinking about something… I'm sure most people have heard that deleting files isn't as simple as emptying the bin, that to be sure you have to zero the disk several times or physically destroy it.
So what if you had say, a full 100GB drive - 10GB for the system and 90GB for files, you delete the 90GB by simply dumping in the trash and emptying it. Then you refill the drive with another 90GB of files, if the original files were not truly deleted does this mean that if you had specialist retrieval software that the drive now holds 180GB worth of files?

 

[spinnerlys]

No, as the 90GB of previous data will be overwritten by the 90GB of new data, and as the HDD only has a capacity of 100GB, where should the additional 80GB come from?

 

[Grannyville7989]

Disk Utility has an option to securely zero out the free space or the entire drive with a 1, 7 or 35 pass wipe.

 

[Jezae]

where should the additional 80GB come from?

That's what I thought, so what's the benefit of 'zeroing' your drive more than once if the first run overwrites all data?

 

[spinnerlys]

Disk Utility has an option to securely zero out the free space or the entire drive with a 1, 7 or 35 pass wipe.

Though that overwrites the data without leaving data after completion, it is almost the same as emptying the Trash and refilling the HDD with the same amount of data that has been previously deleted.

That's what I thought, so what's the benefit of 'zeroing' your drive more than once if the first run overwrites all data?

To prevent others from restoring data without putting actual data on the HDD, maybe when one wants to sell a Mac or internal/external HDD and leave no recoverable data on it.