I ran DBAN with eight passes, and then I ran it with one pass again.
Does that mean my hard drive is as secured as with one pass, or with eight?
No, it's not true. If you overwrite the disk 8 times, and then you overwrite it 1 more time, that's 9 passes total.
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The way DBAN works is it generates pseudo random numbers and writes them to the disk. Depending on the overwrite method used it simply changes the pattern of the writes. What this does is prevent the reconstruction of the data.
Running DBAN with 8 passes and then with 1 additional pass is does nothing different than just running it 9 times.
For example, using the DoD 5220.22-M standard in 3-pass method would write the following:
Now if you were to run it again with just 1 pass you will get:
So basically you've just re-written 0's over the data you already wrote over.
Also the NIST has stated that with today's current hard drive standards that one pass is actually sufficient to render the data near impossible to recover.