If your pictures are not RAW but compressed (say jpgs) then the corrupted photos will really jump out at you - half the picture is there, but then noise. I used to make sure I was storing jpgs not RAW, but then learned that wasn't always a good thing, several times over. Dunno whether damage happened in RAM or on HD, but I strongly suspect RAM.

The bad habit that nailed me was probably this one: I would ball up a whole bunch of photos in a big .zip; (to enhance copying speed and maybe privacy) then next year unzip that, add that year's selected photos and zip it again. Year after year. That added up to a lot of trips through RAM in which one bit flip could take out an image. Bugs in compression software aren't impossible as the villan, but that's not my bet.

Solution: I now zip up a few years together and copy each chunk I don't ever rezip old large chunks. No new errors noticed.