So you’re getting rid of some old hard drives, but you’re nervous about just throwing them in the trash. If the idea of creating a USB boot key doesn’t frighten you, then you should know about Darik’s Boot and Nuke. Simply deleting files or formatting the drive doesn’t actually erase the files on the drive. You need to overwrite every sector on the drive multiple times to really be sure.
Reboot the computer with either a CD or USB key inserted and you’ll boot into DBAN. From there you can choose a number of options for wiping the drive that range from writing 0’s one time across the whole drive to doing much deeper scrubs with multiple passes. I generally default to “DoD Short Wipe” which does three passes. That could take you a day or two with today’s larger drives, but it’s nice to know that nobody is going to find my old hard drive and pick any data off it.
I’ve also been known to physically take drives apart and smash the platters with a hammer (wear safety glasses!) That’s messy and technically it’s still possible to read data if the pieces are big enough, but it’s good enough if I’m in a hurry.
P.S. If you find yourself doing stuff like this often, consider picking up a hard drive dock. It’s a lot easier to swap drives in a dock then to physically open your computer. This is the one I’ve had for a while, but you might want to consider a USB3.0 model too.