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There are two categories of disk, which can be characterized by their structure.
Most distinguishing mark is the case. The older disks got a flexible and soft case. These are called floppy disks.
The newer ones are the so-called shell or cartridge disks. The case of floppy disks contains of a thin and flexible plastic layer, which protects the data layer beneath. These cases were used at the formats of 8 inch and 5.25 inch. At this kind of cases there is a hole to guarantee access to data layer.
The data layer is made of plastic with one magnetic layer of iron oxide.
The data layer is divided in several magnetic tracks. First there were 35, later typically 40 or 80 tracks. Each track is divided in sectors or blocks.
At the format 3.5 inch, disks were produced with the more stable shell or cartridge cases. These disks were not flexible. The data layer was protected by a metallic shutter. The shutter was opened only to access the data layer inside a drive.
Another distinguishing mark is the structure of the data layers. There are single sided and double sided disks. That means you can write on one side or both sides of
data layers. Specific feature of double sided disks: the drive needs two read-write-heads to access the data layer.