At least until I found this very instructive article, “Disk Wiping – One pass is enough“. Don’t miss the second part which clarifies some points and gives more details.

In short, after one pass, every bit of the disk is filled with zero and there is simply no way to find out what the previous value was. Even the best tools out there have no clue to do it.

Then, there is a theory of physically restoring each bit using a magnetic force microscope. It has always came with a high error rate, and with modern high density disks it is even less reliable. Now, considering any real world data length, errors occurring on the restored bits would make it impossible to rebuild any usable data. There is obviously no chance for such a technique to recover a file.

So, in the future, I will not only save time doing one pass, but I will replace :

$ dd if=/dev/urandom of=/dev/sda

with

$ dd if=/dev/zero of=/dev/sda

Note that formating just reset the partition table. In no way it clears out every bit of the disk.