The magnet in the sub is not at fault.

There are two possible reasons why your HDD died:

1. AGE.

2. Vibration.

A drive can die at any time, and they like to pick the worst times. Did I ever mention the time about six or seven years ago IIRC when I had a (purchased new)120GB Samsung HDD in my primary desktop fail mere hours before a scheduled replacement? (250GB Barracuda 7200.something, also new, still works today.)

Or the time a spankin new 1.5TB Cuba LP died juuust after a long format? Just at the point where it is like "ok, it seems to work lets load some data on this thing"

Better put new caps on that HDD.