Erasing Hard Drive question? (Gutmann algorithm)

[MacCrank]

When selling on an old unwanted computer I always clear the hard drive using apples facilities via Disk Utility and erasing the HD choosing 1 of the 3 options: 1 pass, 7 pass and 35 pass erase.

Question:

If one of the 3 above is performed more than once, does this equally increase security? For example, if I use the 35 pass erase twice, (not sure if it would allow me if I did it immediately one after the other would it?) but would this be equal to a 70 (35+35 erase)? The more time performed the less likely to retrieve?

Its a question that has always made me inquisitive and have never found it asked or answered.

Be interested to hear your thoughts

 

[Hellhammer]

Well, performing 35-pass erase twice means that the drive is overwritten with zeros for 70 times, so essentially it would be a 70-pass erase. However, studies have shown that it's enough to secure erase the drive once, it will destroy all data.

 

[MacCrank]

Well, performing 35-pass erase twice means that the drive is overwritten with zeros for 70 times, so essentially it would be a 70-pass erase. However, studies have shown that it's enough to secure erase the drive once, it will destroy all data.

Thanks for the response.

So would you say by performing twice or more, it would double the chance of failing to read data or would it be using identical overwriting patterns each time, creating no change from the previous erase?

You say studies have shown it will destroy all data when the Gutmann is performed only once. Its never destroyed as such is it? I thought it is always assessable with the right equipment and many man hours on hand?

 

[Bear]

...
You say studies have shown it will destroy all data when the Gutmann is performed only once. Its never destroyed as such is it? I thought it is always assessable with the right equipment and many man hours on hand?

One pass erase will be enough to prevent your data being recovered by most people. It takes rather expensive equipment to recover data from an erased disk.

And truthfully if the people who can afford that want your data, it doesn't much matter. However, if it will make you feel better, go for the 3 or 7 pass option (depending on OS version).

Also, the amount of time it takes to erase the disk would be ridiculous for more than 3 or 7 passes.