North wrote:

In these cases do the fdisk twice (fdisk then reformat then fdisk again then
reformat then reinstall the OS).

I use "autoclave", which is no longer offered. The people who wrote and
supported it now recommend Darik's Boot and Nuke:

http://dban.sourceforge.net/

I have not tried that.

With Autoclave, I just boot to floppy and kick back, usually using a
single write with 0, rather than multiple over writes. Then go through
the normal format and install process.

No fuss, and no left over code contaminating whatever is installed.