I suspect there may be a hidden partition on the drive, that has the OS loaded onto it.
The safest way to make sure that there is absolutely nothing on the drives is to <b>low level format</b> it. Have a look on the manufacturer’s website, and see if there is a utility there to do that. This is the step before the Windows format, and will actually write the sectors to the disk, so it will really kill anything that is there before.
I am sure other people will also have advice on this, so you may want to also try their options.