A couple of years ago I gave an external hard drive that I no longer used to my daughter Emily. Before I gave it up, I erased all the files and reformatted the drive at least once.
Recently the same hard drive became unreadable and Emily asked me to help recover her files.
Lesson 1: Never rely on one copy of ANY electronic files!
The file structure was unreadable, so I went to brute force, reading the physical hard drive file-by-file. Over 100,000 files were recovered.
Including some of mine from years ago!
Lesson 2: Erasing a disk does not erase the files — unless you use special security erasure, and even then perhaps not completely.
The recovered files do not have their former names. We have 100,000+ files with random numerical names. The files do have their extension — pdf, doc, jpg, etc.
Lesson 3: Recovering lost data is a tedious task.
Last time I wanted to get rid of a hard drive I decided to erase it, then erase it again with a belt sander.
You should have three copies of every important digital file. And the three should be on three different hard drives. And one of the three hard drives should be stored offsite (or at least in a fire-proof safe).
You can trust me on this, I am an archivist.