The metal disks that are permanently housed in sealed and contamination-free containers are called

Metal disks permanently sealed in contamination-free containers are hard disks.

Floppy disks were flexible and kept in open plastic cases, not sealed.

Optical disks use light rather than magnetic metal platters.

Flexible disks are just another name for floppy disks.

Hard disks store large amounts of data on spinning magnetic platters.

Their sealed casing keeps out dust that could damage the surface.