|Even after a drive has been wiped multiple time it is still possible to recover data from the residual magnetic field. It is a complicated procedure to completely erase a hard disk beyond recovery wiping various times with different bit patterns. One pass of kill disk wont do it.|
With a hard drive that has really sensitive information on it where it may be worth the time and effort to recover the data then only way to ensure that cannot happen is to take a sledge hammer to the platters.
When a tape is erased a bias frequency is written to the tape which effectively masks the information that is there. A bit like an unmodulated carrier wave. Slight variations in the bias frequency from one machine to another can allow the old information to be recovered.
With the right equipment the bias frequency can be adjusted to match exactly the frequency of the original recording and filter out any other frequencies that might also be there. A bit like tuning a weak HF Single Side Band radio signal when there is stronger single present.
When tape was normal in recording studios, master tapes were always generated from brand new tape. You would never make a master tape from a tape that had been previously used because the previous information is never completely obliterated.