BTW, I don't think most who either sale or donate used computers actually take the time to securely delete personal info from storage devices. Many just do a full format, same as a quick format with surface scan, and assume their personal data is safe. Some even re-partition the HD to free space. Both do little to actually prevent personal info from being recovered.
In fact, I bought a used external USB hard drive that the owner both re-partitioned and re-formatted. Out of curiosity, I wanted to see if any files could be recovered.
I was able to recover over a thousand files. Many family photos and some pdf and doc files. Enough so that I could assume that the previous owner was an engineer.
An easy way to prevent this, not to mention the quickest way to securely delete the contents, is to encrypt the files. Then, when you don't need it anymore, just delete the hard drive contents. You're done. Any files recovered is just garbage of unintelligible characters without the encryption key. This also saves a huge amount of time from securely wiping the drive after the fact.