I'm sure you found the right magnet, it's on what I believe is the head assembly. Whne you get the HDD case open (for everyones reference) you'll see it resembles a record player. The actual plates where the data is stored is not where the magnets are. They are located where the "arm" pivots, in the corner of the case.
1" isn't too small, all factory magnets I've seen, and magnetic drain plugs, are all smaller than this, and weaker to boot. I snapped one onto my oil filter. If I had a bunch of them, I'd snap a few onto it if I was really worried about it.