The best method to prevent others from reading the data on hard drives is to destroy the drives physically: a sledgehammer is a cheap and effective tool to prevent the data from being read again. Since many types of hardware, including hard drives, contain toxic materials, it'd be a good idea to take them to some responsible e-waste recycling services.
There's a lot of info on the web about diverting e-waste from landfills. There's also plenty of discussion on destroying data on hard drives. But I have not found info on ensuring the data on the drives has been destroyed before giving them to the e-waste recycling services. I afraid that a hard drive that has been beat up too badly becomes difficult or impossible to handle by the recycling services.
Is it possible to satisfy both the environment and my own privacy?