13. Floppy disks literally get mouldy. The magnetic media attracts spores when disks are stored in an attic or garage that is warm and humid. I have no idea whether the spores are attracted to the iron or similar in the floppy media, or to other aspects of a floppy disk.

The first time that I observed the problem was when making backups of twenty year old Apple II disks. I ended up using a bare Apple II drive on the bench for "source" to make copies.