I reformat any drive, then print and scan 10 test pages to and from my laptop with a crossover cable.
I delete or restore a blank address book backup.

Reformat again.

Copiers are limited to a 10 document queue that overwrites the last 10 jobs.

Even if a lowlife hacker searches the HDD, there is nothing more than test prints and scans.
Time used, about 30 minutes and HDD is clean.