|You would need to read about the progression of hard drive capacity. There is something called areal density. As the areal density grows the head alignment becomes more critical. |
IMO you are right about dependability. I think it is a combination of the density and quality control. Look at the link below and also look at the specs of any hard drive you are considering. The more data per platter side the higher the density. This makes reading the data faster but as I mentioned, things can get out of wack easier.
There was a firmware issue with one brand and series of drives recently that virtually guaranteed the drive would fail to retain data.
All that said, there is no substitution for regular backups.