I've heard a number of things about magnetic media slowly going bad from
external fields, but it's not true.

Unless you exceed a certain minimum field strength, you won't affect the
data.
This can be shown by taking two credit cards and rubbing the magnetic strips
together. Myth says that this will erase them,. but the fact is that the
residual magnetism in any media has to be less than the threshold to change
that same media.

Now the oxide may fall off because the binder goes bad, or the mylar may get
brittle with age, but that's a different story.

I've seen people flip out over a megnet used to hold a note on a PC case,
not realizing that there's a set of magnets far more powerful, inside every
hard drive, right next to the platters.
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