There are a couple of approaches that you can take in disposing of old hard drives. I have a stack of hard drives I need to dispose of.
What you can do is this: I have a USB adapter. There's a USB cable available (actually there's several version of these things), but what they are is a USB to either SATA or IDE connector cable. Using a USB connector
The non-USB end of this cable just looks like this big plastic double-sided connector thing. What that allows you to do is to basically turn any external drive, any hard drive that you happen to have, into a USB external drive. You simply connect it up to the right connector on that cable, potentially provide power (which most of the cable packages also provide) and then access the drive as a USB external drive on any computer you happen to have available. Wipe the drive
Then if you haven't already extracted your data from the drives then you can run something like Darik's Boot And Nuke or DBan for short on it - but it's not really necessary. What you can run instead is CCleaner CCleaner - PC Optimization and Cleaning - Free Download
which has a wipe disk option.
You can actually tell CCleaner to go ahead and delete everything on this external drive and it will simply do it. It will take a little bit of time. If you like, I believe it actually has the option to overwrite more than once. In my opinion, once is plenty - especially on an older drive.
That then should leave you fairly confident that if there was anything that could have been recovered on that drive... it's not going to get recovered anymore. Dispose of the drives
As to what to do physically with the drive, I honestly don't have an answer for you because that's one of those things that varies dramatically from location to location - and that's what I would do.
Itís illegal to toss them in the garbage or a dumpster in the U.S. as far as I know.
I would look locally. I would actually check with some of the local computer stores and see if they have any recommendation for PC recycling facilities and that can accept your donation of these hard drives. Electronic recycling centers usually set up a day where you can bring any electronic items (usually at a business neighborhood store) where you can bring anything electronic and dispose of the items.
If they're very old, they're probably not good for anything anymore. That's kind of unfortunate, but at least that way, you can get it into the hands of somebody who will properly, in an environmentally friendly way, dispose of the electronics associated with the device.
On somewhat newer drives, like maybe within the past ten years or so, the drives may still be valuable to someone. Hopefully, there will be a way for your recycling efforts to make those drives available to those who need them.
But that again is something that you would have to look at locally for - rather than try to come up with a generic solution that would work country-wide or even planet-wide.