I've never found enough information to actually contact the previous owner of discarded computer or HDD, and if I ever do, I don't think I'd try to, for the same reasons you mention.

I've had tons of opportunities, but never attempted to contact any of the previous owners.

Why not? Because it's of no use, the computers I disassembled on the sidecurbs were trashed so I had several reasons to not return the stuff I found.

First of all, the owner tossed their own computer, so it was their own choice to put it on the sidecurb in the first place.
Second reason was that, as this was at the time my only way to get hardware cheaply and relatively easily, if I would've returned all the harddrives I found, the previous owners would likely just thank me for returning their data, close the door in front of me, smash the harddrive with a hammer and then put it on the sidecurb for a second time, so the only thing different would be that I wouldn't have aquired any of those harddrives.

As that was of no use to me, I never even had any motivation whatsoever to return the data to the previous owner. And last but not least, I wanted people to put their stuff on the streets so I would continue to find free stuff I could take home with me 😁

When I bought Libretto 70CT, the HDD was full of stuff left by a German professor of psychiatry: personal letters, agendas, articles. Apparently, he was also lecturing with the machine. It was easy to find his email there. I wrote him that I have bought his laptop and would like to send him his stuff over internet before "erasing the hard drive". Unfortunately, he did not respond. He might be dead, btw. I still did not format the hard drive 😀