a mag, no, friends who had them yes.
http://www.dansdata.com/gz009.htm
and
http://www.dansdata.com/magnets.htm
a decent exposure.
to this I add a few things,
i have a few rare earth magnets, most from hard drives,
and they do generate a great deal of magnetic force at close range, considerably larger than your average pump.
If you have an old monitor you can do the test, the distortion of the picture caused by any of both is different, and aparent. Or murk about with them
on an old hard drive, nothing really happens, and nothing should. I can state this for a fact.
Also to the potpourrie , a MAG driven pump has a smaller EMI than a brush motor pump. It can't even interfere with a floppy drive, let alone a Hard Drive.
side note: eheims have some EMI shield AFAIK.
From my (very limited, i admit) experience, again, let me restate,
as i think you misdiagnosted your problems, very common
no pump i know generates a EM field so powerful that could actually harm a HD in that way.
not calling anyone a liar, just find that very hard to believe.
just for the record, which hard drives?
IBM Deathstar?
Seagate "narwhal, barracuda my ass" ?
PS: the maxijet does her fair share of EMI. Mussed my monitor even at a good lenght from it ... and killed no hard drive.
PPS: this is Procooling, being nice is not normal policy
.