In your case, there're three possibilities:
i- the magnetic field had indeed caused damage as you yourself reckoned;
ii- most of the disks had already gone bad since they were left in closet for years;
iii- the disk drive in your A1200 is no longer mechanically sound since it's been left unused for long time, as suggested by ddniUK.
There's always another option:
iv- you're dealing with an old-school virus, and you just applied it to the rest of your disks... viruses don't just magically die off when you store disks. Its rare that I'll go through a stack of old disks without something unexpected being on one.
