A little late to reply, but this thread was too good to avoid and I'd like to share my experiences with this since I've collected (probably way too many) machines I my time.
I don't like to make a habit of prying into other's personal affairs, and I don't judge anyone here at all. However the times curiosity have gotten the better of me, it has been mostly mundane stuff.
About a year and a half ago, I bought an Amstrad PC1640 from an older gent locally, and he really treasured this machine. Nothing much in the way of personal data, but he had a journal in which he logged everything he did with that machine. Software and hardware wise. Also his PC magazine subscriptions. Really was quite something. And yes I definitely will keep that book with the machine.
In different cases, there's been few times there were banking info and addresses, nothing really worth mentioning. Though, my most interesting data dive is from a Mac funny enough...
A few years ago I received a Classic II that was not working (mess on the screen). I cleaned the main board with the usual soapy warm water trick, a total cheap-out fix and got it booting again to find it had been used as a family computer with records upto 2002.
It wasn't exactly chock full of sensitive data but what I have determined is that the family that had it, the kids seemed to use it most to do drawing and typing in Claris Works. School stuff, love declorations (to various girls at their school lol), cartoonish faces, etc. Typical innocent kids stuff.
The mother seemed to use it for logging her chiropractic work (presumably self-employed), writing invoices for people owing. She wasnt cheap either, upto £200 a session, in the early 2000's!
There were a few miscellaneous letters, one about their divorce which was kinda personal obviously. I didn't read into that much. But I think the one that almost brought a tear to one's eye was a number of letters one of the kids wrote to their devorced dad.
Reading it I was seeing how distant they were, not seeing eachother for nearly half a decade, talking about his schooling and football game results and mentioning that he wanted "to be a policeman just like him". Also wishing he could contact him more often.
I do wonder if his mother took a dislike to her son making letters to her ex husband as he kept apologising for not responding more regularly.
It was quite moving personally, dealing with a family breakup myself when I was about 7, I suppose I was lucky I got to see my dad fairly regularly as a kid.
It really pained a picture of the machine's past. I even discovered a faint UV mark on the screen bezel, a completely different named individual. One assumes this was either sold on or given to this family (prior to being wiped presumably) in the late 90's as their earliest document date was around 1998.
I'm not really proud of myself for looking, but it does really fascinate me about what computers I have and what story they have to tell of their past.