“However, the data has been written to the bad sector (admittedly, one or two bits of it may be wrong, but this leaves more than 4000 genuine bits) and since the sector is now marked as “bad”, it is forever inaccessible from the host computer. No amount of wiping can do anything about that.” – the ATA command enhanced SECURE ERASE will over-write blocks marked bad. [1]

“The net result is that the new data may be physically off the previous one by a small bit; the old data is still readable “on the edge”.” – this is at most a theoretical risk, but really it’s not useful paranoia. Here’s what someone else says about it: [2]

One of the chief controversies is that if a head positioning system is not exact enough, new data written to a drive may not be written back to the precise location of the original data. This track misalignment is argued to make possible the process of identifying traces of data from earlier magnetic patterns alongside the current track. This was the case with high capacity floppy diskette drives, which have a rudimentary position mechanism. > This was at the bit level and testing did not consider the accumulated error.

The basis of this belief is a presupposition is that when a one (1) is written to disk the actual effect is closer to obtaining a 0.95 when a zero (0) is overwritten with one (1), and a 1.05 when one (1) is overwritten with one (1). This we can show is false and that in fact, there is a distribution based on the density plots that supports the contention that the differential in write patterns is too great to allow for the recovery of overwritten data.

“The basic conclusion is that wiping does not work against a determined attacker.” – this is true if you’re talking about SSD or other flash type drives, but it’s not true if you’re talking about conventional hard drives.

“Bottom-line: media destruction requires resources. In a business environment, this could be a system administrator task, but it will involve extra manpower, safety issues (seriously, a geeky system administrator with access to an acid cauldron or a furnace, isn’t it a bit scary ?) and possibly environmental considerations.” – This comment is gently baffling. There are companies who will come to your site to shred your drives. The cost compares favourably with employing staff to sit and wipe drives (which takes a long time) and it gives re-assurance to people who believe popular myths about being able to “recover blobby bits” from over-written drives.

[1] http://www.t10.org/t13/project/d1321r3-ATA-ATAPI-5.pdf [2] http://www.vidarholen.net/~vidar/overwriting_hard_drive_data.pdf