Part of this was the fact I have recently acquired a used piece of hardware. Totally amazed at the amount of personal info that was left on it.
Pretty much the exact same thing happened to me about three years ago... I bought an external hard drive at a yard sale for $6. The seller said it hadn't worked in months, and claimed not to know what it had been used for. The housing was cracked, and the power supply was junk, but I though I could salvage it.
Stripped it out, hooked it up to power and eSATA, and whaddyaknow, a working 1Tb drive about 30% full of very private info: financial records, investment meeting notes, spreadsheets detailing years of real estate dealings, lists of bank accounts, drafts of wills, etc.
I was fairly stunned. Should I return the drive to the owner? Should I just delete it? What if the info was important to them, and not backed-up elsewhere? Would they be angry, wondering what I might have done with the info? Would they believe that I hadn't copied it? Would telling them either way just embarrass them? I didn't know them, but they're a couple I see around town often enough, and every time I saw them, I'd cringe a little inside.
Ultimately, I decided to do what they should have done in the first place and wiped the drive with a low-level format before properly re-purposing it. The same drive is still in my media server as an ext4 volume full of movies.
I never told the previous owners. At best, I'd be educating them. At worst, I'd be opening myself up to accusations and hassle. In any event, people would be getting seriously stressed over something easily avoidable. I guess that's how I'd like it to go down if the shoe was on the other foot.