Here’s an expanded comment I posted to Computerworld recently. Re-using an old hard drive yourself, by putting it in an external drive enclosure and using it for backup is fine. (I have one in a safe deposit box, in case my house burns down). But don’t donate it, sell it, or even recycle it unless you destroy the data on it properly first.  RB

Recycle an Old Laptop Hard Drive

RB comment: Just be sure to destroy the drive if you are finished with it. Unfortunately, donating it /selling it are not wise. You can also do a “secure erase,” which encrypts the old data and allows the drive to be safely reused. Reformatting the drive, and running the various utilities that supposedly overwrite old data, are NOT substitutes, because they don’t get at the underlying data thoroughly. And tests of drives purchased on eBay still show about half of them have proprietary data, including financial records etc.

Here is one such academic study: Remembrance of Data Passed: A Study of Disk Sanitization Practices. There are many others.   Here’s a good popular article.

Here is an explanation of safe erasure. http://cmrr.ucsd.edu/people/Hughes/DataSanitizationTutorial.pdf

For physical destruction, the easiest method is a hammer applied to the spindle. Once the bearing has “wobble” in it, the drive can’t read the tracks any more, and it would take an NSA-level lab to recover data, even partially. You can also smash the circuit card. It can be replaced, but the thief has to work much harder to find a compatible card.