i would imagine if you are using a computer sub you shouldnt worry too much. i would think they are shielded or using a bucking magnet.
i regular sub in a small box....with a hdd in very close proximity i would worry. i dont think i would worry very much if it wasnt sitting RIGHT on top of it.....
now with that laptop story...i would have to think that laptop hdds are not shielded as well as regular hdds.
bottom line, use common sense. regular home/car audio sub + hdd in close proximity = potential danger. but you shouldnt need to be too paranoid about it, just keep your hdd away from it. it wont hurt to have the sub 'in the general vicinity', just avoid close proximity. mount your hdd up high.