I had 2 experiences with wiping HDDs recently, one planned and one unexpected. This is what I found out.
I used Darik's Boot and Nuke, a free software that has several options
for wiping. The problem (or advantage) is that it's boot software, so
you need to make a boot disk. The easiest way I found was to use Universal USB Installer (google both togetherfor instructions).
Another problem was that I kept getting dev sda sdb errors. Searching
online showed these were caused by additional devices like card readers, but people with these errors usually had strange drives show up in DBaN, I only saw 2 HDDs and the USB disk. Anyway,
I took out the optical and HDD from my laptop, but couldn't figure out
how to disable the card reader. This still worked, so that's good