search around... information density on today's disks are so high that one pass overwrites pretty much everything, and any bit left is too thin to be of any use
back on the IDE disks time, a electron microscope could in theory recover some information, but that was a lot of time ago...
and I never heard of multiple layers of unindexed data on a HDD... any reference?
dd - is the killer if=/dev/zero - gets a stream of zeros of=/dev/sda - if your hdd is sda, it will be zeroed out bs=4M - copy 4mb blocks at a time oflag=direct - write straight to the disk
only make sure /dev/sda IS the disk you want to kill
the rest can be as it is
make really sure, make sure ten times over... once you start, it's too late to regret
killall sends the kill signal, the USR1 is a kind of signal that does not kills
*sends the kill signal by default, but you can send another one, and USR1 is a signal that can do anything the developer wants... on the dd case, it will print status messages
"kill" is a bit of a misnomer, because what it actually does is send a signal to a process. The default signal is SIGTERM, telling the process to please end. The more brutal variant is SIGKILL, signalling the process that it is now being killed. -USR1 sends the signal SIGUSR1, which just means "some user-defined signal", which in dd's case means "display the progress".