Not that "hammer method," or, in the case of laptop and compact external HDDs, " drop it on to the sidewalk from head height, preferably several times method," will not both work, but there are many occasions where someone may not wish to destroy a HDD as part of the bargain.
Since I know that there are folks who'd like to pass their old computers on to friends or relatives or donate them, but don't want their data going with them, I will present my technique for data cleansing a computer that's been used.
1. Create a new local account (Win10) or just account (earlier Windows) with admin privileges.
2. Log in to that newly created account and use it to remove all other existing accounts. Do not, of course, save the data from same to the desktop, as you're trying to get rid of it.
3. Get CCleaner from piriform.com and install it if it's not already on your system.
4. Open CCleaner, choose Tools from the controls column on the left, then Drive Wiper from the tools list presented.
5. Accept the "Free Space Only" option, as that's where any of your old data of any significance is now located, and choose the Security setting based on your level of paranoia about the ability of an individual skilled in forensic recovery being able to recover data from a drive that's been wiped. This would generally mean the 3-pass overwrite would be way more than sufficient, 7-pass almost certain to be beyond anyone's reach, and 35-pass almost being "the NSA couldn't get stuff back from this system's former user data areas."
6. When the wiper has completed, for most folks (and I include myself in that, as that's where I stop) - you're done.
7. If you now want to be incredibly anal retentive do either a recovery back to "out of the box state" using the recovery partition that's been included on most computers for some time now or, if you're a Windows 8 or 10 user, a completely clean reinstall of those OSes. Overkill in my opinion, but if it makes you sleep better, more power to you.
And, while I am sure that someone is going to come back and tell me that the NSA could recover something, I'm truly not worried that my used computer is ever going to be of interest to the NSA nor to come into the hands of someone with NSA-esque forensic recovery skills. I prepare for the probable scenarios, not those so remotely possible as to not warrant serious consideration.
Brian AKA Bri the Tech Guy (website in my user profile) - Windows 10 Home, 64-Bit, Version 1809, Build 17763
Travel is fatal to prejudice, bigotry, and narrow-mindedness, and many of our people need it sorely on these accounts. Broad, wholesome, charitable views of men and things cannot be acquired by vegetating in one little corner of the earth all one's lifetime.
~ Mark Twain