I have read in several places, I don't remember exactly where, that to effectively and securely erase stored date from most modern digital storage devices, it is necessary (or at least this is one often-used technique) to repeatedly, in several, maybe dozens, of passes, write randomly selected strings of 0's and 1's over the data. My Bitdefender antivirus app now has what is called a "file shredder" to do just that. (I imagined from the name that it used this technique, and a call to Bitdefender indicated that it probably did.) My question is: Why is it necessary to use such a time-consuming technique, when, it seems, just writing all 0's (or all 1's) would be quicker and even more effective? It is true that the ultimate limit set by quantum mechanics on the accuracy with which the state of every atom, free electron, etc. in the storage device after writing the 0's could be known would allow, at least in some situations and for some storage locations, the determination of whether a 0 or a 1 had been stored there before writing the 0's, but this could not be done with present technology, or any likely future technology. I could be wrong about this. Am I wrong, or were instead the descriptions I read of erasing techniques, and the Bitdefender phone tech person, both wrong, or, more unlikely, are the erasing techniques used generally, including by Bitdefender, obviously needlessly complicated and time-consuming?