Hi,
I’ve heard that the thing most likely to go bad on a hard drive that is stored for a long time is the bearings. But I would guess that the motor could also go bad. The actual discs are just as good as tape though as far as magnetic storage mediums go.
Eventually the magnetic field of the earth will erase or drastically degrade any magnetic recording medium, with the exception of magneto optical, I believe.
With how cheap hard drives are nowadays I would suggest just planning on moving data from old archive drives to new drives every couple of years. In order to save money you could even re-use the old archive drives to backup stuff you don’t care as much about.
Good luck,
Tim