Why do good floppy disks go bad?

[JP Hindin]

I had one of those Japanese Koan moments recently when someone asked me 
"Why do floppy disks stop working?" and I realised I... didn't actually 
know. I thought I'd throw it to the group and get some theories/proofs.

Let's work on the assumption we're talking about 5.25" and 3.5" disks.

Several guesses:
- Repeated use slowly wears away the magnetic media layer on the mylar.
- When left in an unprotected state, or a poor environment, damp, mold and 
dust can damage the surface, either degrading the magnetic layer or 
causing the gap to shrink enough that the drive head physically damages 
the disk?
- Quantum fluctuations in the state of the universe, caused by millions of 
mostly non-interacting particles passing through a disk in any given 
minute, alter the magnetic spin of the ferric atoms causing gradual data 
loss over time (mostly tongue-in-cheek)
- Given the lack of use of most floppy drives they themselves pick up 
'gunk' and on first reading a diskette after a long time of disuse damage 
it.

It _seems_ like when you put a 3.5" disk down for ten years and pick it 
back up, a disk that used to work fine no longer does. Of course, after 
ten years, it could be your own memory that's failed.

Dare I ask, what's the consensus?

  - JP