Everything I know about floppy disks
Interesting article. I immediately thought of one (very obscure) thing I know about floppy disks which the author appears not to – &quot;deleted sectors&quot;.<p>Before the data of each floppy sector, is a sector header. Among other information, it contains a flag byte, which can have two different values, indicating two different types of sectors – normal and deleted. There are different commands to read&#x2F;write normal sectors vs deleted sectors. Historically, this came from the IBM 3740 data entry system, where each (128 byte) floppy disk sector stored one database record, there was not much in the way of a filesystem, and you would delete database records by changing a normal sector into a deleted one. Subsequent systems (such as IBM PC floppy controllers) retained this feature despite it being useless for their purposes. Almost no microcomputer&#x2F;PC software ever used it, the principal exception being that it saw repeated (ab)use to implement an (easily circumventable) form of copy protection. Apparently, some late model PC floppy controllers (1990s &#x2F; early 2000s, just before floppy drives disappeared entirely) dropped support for deleted sectors completely, viewing it as a disused legacy feature which was a waste of circuitry to support.<p>Well, that&#x27;s true for any floppy based on the traditional IBM minicomputer&#x2F;mainframe floppy format, which includes IBM PCs and many CP&#x2F;M systems. It isn&#x27;t true for floppy disk formats which lack that IBM heritage, for example Apple II floppies.