You appear to have fallen to the age-old (or rather, about 15 year old) myth that there is a readable 'echo' of a bit after it has been overwritten. This is false, and has been ever since the magneto-resistive head was introduced (and especially the GMR head around 2000). Even with a magnetic force microscope you're not going to be reading squat other than the current bit value (or rather, the current analogue value that would then have to be processed along with the preceding and following values into what is probably the bit that you wrote). While it may have been true - in the days when a R/W head actually had a little coil of wire in it and capacities were measured in megabytes - that you could throw a few hundred engineers at a platter with a MFM and maybe recover a little bit of coherent data, that is no longer true. The amount of data you'd need to comb through if you could recover it perfectly (which you can't) is so immense that you'd need an army working 24/7 for decades. Even if you knew the precise data you were looking for and it's precise location on the platter, if you wrote a single zero pass over that location no MFM would be able to read anything useful, or the same head that was used in the MHM would be used in HDDs and you;d be back to square one.
The ATA SECURE ERASE command tells the head to write zeroes over the entire surface of the platter. Regular write-random-crap-a-bunch-of-times software such as DBAN fail to erase sectors in the G-list, which could subsequently be recovered (albeit unlikely to contain anything interesting). it won't erase sectors in the factory P-list, but nothing was ever written there in the first place. This is sufficient to make any data that was on the drive totally unrecoverable. Hell, ATA SECURE ERASE is NIST-approved for government and military data destruction at the same level as physical destruction of the platters.