I keep reading in these forums that the only way to make sure the information on a hard drive is unretrievable is to remove the drive and destroy it. Is this true? And, if so, what is the technical reason a drive is so hard to clean? thank you

Not strictly true, although simply deleting data does not actually destroy it, data can be overwritten to the point where it is basically unrecoverable. Undeleting files is fairly simple if they haven't been overwritten - otherwise, recovery is only likely in the hands of forensic experts. Killdisk (for one) If you get people asking the wrong questions, you don't have to worry about the answers

Read what Rutgers University wrote on the web: Departments' Physical Destruction of Media. There are highly specialized recovery programs that are able to recover data even after more than 100 overwrites. If your data security level fall into a category needing protection from this type of recovery IP&S recommends that the media be physically destroyed and the pieces disposed of into at least 2 different landfills. (home and work). However, physical destruction of hard disks is difficult and may cause injuries. Drilling several holes in a hard drive is not acceptable. i_XpUser

Where there's a will (and sufficient expertise) there's a way, sure. I suspect more useable data would survive on a drive that had only been physically damaged than on one overwritten to DoD standards - you'd still need the proper equipment though Yes, paranoids sometimes do have real enemies, but it's highly doubtful a forensic team will be unleashed on the average person's discarded HDD If you get people asking the wrong questions, you don't have to worry about the answers