(B) To correctly clean a working hard drive use software that does multiple overwrites as it erases. "Tune Up Shredder" is the program that I use. I set it to do triple overwrites, but there are other programs that will do the job just as well.
dd if=/dev/urandom of=<harddrive>
dd if=/dev/zero of=<harddrive>
(in that order) ... the newly zero filled drive will need to be repartitioned before os of choice can be installed...
...im not quite sure about that one in ten thing... every
old comp and cell phone ive come across in second hand stores still has the original owners info... could care less about it... unless there is data on the hdd that is mandatory for the computers operation i wipe it completely clean... others arent so 'honest' ....
(example of mandatory data would be the cmos setup stored at the beginning of the hdd in old compaq systems or otherwise unfindable device drivers and setup/format utilities for MFM harddrives)